Natacha Aveline-Dubach
email: natacha.aveline@cnrs.fr

The Financialization of Rental Housing in Tokyo Forthcoming in Land Use Policy

Keywords: financialization, securitization, rental housing, Tokyo, REIT, bubble, land use, property development, urban planning

Over the past two decades, the private rental sector has grown significantly in Japan. Once an overlooked sector of the market, it has been seized by the financial industry to the point of becoming the second largest REIT residential market in the world. This paper explores the development of residential REITs in Japan, in a context of demographic decline and urban shrinkage. It highlights the strategies of major Japanese real estate groups to diversify their activities and strengthen their control over popular downtown Tokyo neighbourhoods, building on government initiatives to revitalize land markets and stabilize the banking system through REITs. As the paper shows, the need to secure financial investors' expectations of attractive returns has led REIT asset managers to target the vast majority of their leasing activity to Japan's young, "promising" corporate employees. By pointing to the mediation of large corporations in the landlord-tenant relationship, the paper brings these neglected actors into the framework of financialized rental housing, and puts the analysis into the broader context of employment.

Introduction

During the post-war period, Tokyo's land-use pattern fit Alonso-Muth's market-led monocentric city model remarkably well. Major employment zones of white-collar workers were located in the city's five central wards, while non-competing residential uses expanded towards the outskirts. With the cost of transport offset by company support for employees' railway fares, residential neighborhoods further sprawled under successive waves of land booms. The last speculative drive, known as the "land bubble" (1985)(1986)(1987)(1988)(1989)(1990)(1991), expanded Tokyo's commuting area beyond a 50 kilometer-radius from the center. However, the bust of this wide-ranging bubble in the early 1990s put an end to this orderly, yet unsustainable, pattern of land-use. Residential functions shifted back to the core areas, resulting in a dramatic reconfiguration of the job-housing balance. This process was marked by a surge in the provision of quality rental properties in a market sector that had been previously overlooked by the real estate industry. A major driver of this change has been the rise of institutional financial landlords, specifically, real estate investment trusts (REITs) in Tokyo's residential market.

Recent work on financialization in the private rented sector for housing has highlighted the emergence of a new breed of "global corporate landlords" [START_REF] Beswick | Speculating on London's housing future[END_REF] or "financialized landlords" [START_REF] August | Gentrification, suburban decline, and the financialization of multi-family rental housing: The case of Toronto[END_REF] which have come to manage large-scale portfolios of rental properties in several metropolitan regions of industrialized countries. This strand of literature has highlighted a range of predatory strategies on the part of these financial actors, especially in the post-crisis landscapes following the global financial crisis (GFC), generally framed through David Harvey's conception of (capital) "accumulation by dispossession" [START_REF] Harvey | The 'new imperialism: accumulation by dispossession[END_REF]. Such strategies consist in purchasing depreciated properties with the aim of aggressively increasing rents or speculating on the appreciation of the assets' values. While recognizing the growing importance of predatory behavior, this article moves away from the dominant emphasis on economic eviction, displacement and gentrification, and instead explores a relatively neglected issue, namely the ability of this housing sector to meet changing needs in the context of Japan's economic and demographic transition.

In so doing, the present paper, which is part of a special issue on the financialization of urban space, contributes to debates on the interplay between global finance capital and housing market dynamics, based on the experience of Japanese residential REITs. The empirical grounding of this research in "post-bubble" Tokyo highlights the strong constraints that apply to the circulation of REIT capital in potentially declining real estate markets, and points to the multivarious efforts made by the State to organize the conditions of a dynamic housing market.

It also illuminates the pivotal role of Japanese corporations themselves in reshaping housing needs amid a shrinking workforce and the dismantlement of lifetime employment traditionally found in these companies.

REITs are investment entities comparable to mutual funds that pool capital from small and large investors to acquire and manage portfolios of income-producing properties. As REITs' shares are usually sold as securities on stock markets, the emphasis is put on the securitization of real estate rather than its financialization in a broader sense.

Although some REITs have speculative strategies that are clearly similar to those of short-term focused equity funds (particularly in the residential sector), it is important to take into consideration the restrictive regulations that govern their activities. As suppliers of "defensive" investment vehicles (i.e. offering low risk-adjusted returns), REITs are subject to a series of constraining rules relating, inter allia, to ownership control, leverage ratios and trading activities. These regulations vary between countries, but they have important commonalities that help to promote cross-border investment in the REIT sector.

In Japan, the REIT regulatory framework is quite restrictive. Japanese REITs (hereafter J-REITs) are not allowed to engage in real estate development, and must build up their asset portfolios by purchasing properties. J-REIT asset management structures are generally established by a real estate/construction conglomerate known as a "sponsor group" (of which they are a subsidiary) and tend to purchase the properties from this group (fig. 1). These conditions therefore limit the opportunities to "buy low / rent high" that can be found in other REIT markets.

But more importantly, Japan is an unusual case where an overall decline in real estate values seems inevitable. Against a background of weak economic growth and demographic shrinkage, many parts of the country have already started to experience a structural decrease in housing values, including some areas of Tokyo's suburbs [START_REF] Kubo | Aging suburbs and increasing vacant houses in Japan[END_REF][START_REF] Phelps | Edge City Denied? The Rise and Fall of Tokyo's Outer Suburban "Business Core Cities[END_REF].

While J-REIT's channels can hardly be used for speculative purposes, they offer Japanese real estate conglomerates valuable opportunities for business diversification. At the time of an initial public offering, sponsor groups dispose of part of their properties in one or more REIT platforms in order to mobilize capital "locked" in their real estate holdings. Although they no longer own the transferred properties, the sponsor groups exercise full control over the management of the REIT's assets and receive fees on the real estate portfolio [START_REF] Ooi | The wealth effects of property acquisitions: Evidence from Japanese and Singaporean REITs[END_REF]. Other properties are generally provided by development companies affiliated with the group -but not exclusively -and the building management is also entrusted to a group affiliate.

Within this context, residential J-REITs have recorded substantial growth since their introduction in 2004. They now rank second globally with a capitalization of JPY 2.74 trillion 1 (USD 25 billion), yet still well behind their US counterparts. 2 Currently, no less than 22 J-REITs are involved to varying degrees in the housing sector, managing a total of 1,410 condominium buildings in the Greater Tokyo area. This paper examines the underlying dynamics of this growth from an actor-centred perspective. Focusing on the relationship between real estate conglomerates, J-REITs and the State, it looks at how these actors have joined forces to create an attractive private housing rental market to secure stable demand from solvent tenants while also advancing homeownership policies.

The method involved discussions with a range of actors. Semi-structured interviews were conducted with asset management teams of 12 REITs managing more than 40 residential properties in the Greater Tokyo region. The questions focused on the REITs' locational strategies and modes of asset acquisition, their targeted tenant profiles and the characteristics of the properties. Complementary information on the broader context of the private rental sector in Tokyo and State policies was obtained from in-depth interviews with eight representatives of urban planning associations, leading Japanese real estate developers, real estate associations, and real estate consultancy firms. Data collection has also relied on the use of government reports, the ARES database of J-REIT properties, as well as a review of real estate reports and press articles.

The paper has two major sections. The first combines two bodies of literature, on the securitization of real estate and the rise of financial landlords, to highlight how financial investors and related lobbies have recently succeeded in developing a profitable private residential rental market with the support of State policies. From this, the second section discusses the experience of residential J-REITs in the Greater Tokyo region, examining the combined strategies of State authorities and REIT sponsor groups to extract value from the built environment within this changing demographic context.

Securitization, State Policy and Changing Housing Needs

Real estate securitization refers to the process of converting inherently illiquid commodities into exchange-tradable investment vehicles by way of financial engineering [START_REF] Corpataux | Increased Capital Mobility/Liquidity and its Repercussions at Regional Level Some Lessons from the Experiences of Switzerland and the United Kingdom (1975-2000)[END_REF] and the development of information and communication technologies [START_REF] Lizieri | International property portfolio strategies: problems and opportunities[END_REF]. As such, it is part of the broader transformation of real estate into a "quasi-financial asset" [START_REF] Coakley | The integration of property and financial markets[END_REF], designed to extract value from the built environment [START_REF] Weber | Extracting value from the city: neoliberalism and urban redevelopment[END_REF][START_REF] Corpataux | Increased Capital Mobility/Liquidity and its Repercussions at Regional Level Some Lessons from the Experiences of Switzerland and the United Kingdom (1975-2000)[END_REF][START_REF] Gotham | The Secondary Circuit of Capital Reconsidered: Globalization and the US Real Estate Sector1[END_REF][START_REF] Gotham | Creating liquidity out of spatial fixity: the secondary circuit of capital and the subprime mortgage crisis[END_REF][START_REF] Theurillat | The real estate markets: Players, institutions and territories[END_REF]. Investors benefit in two ways from REIT investment, through regular dividend payments but also through the share value appreciation [START_REF] Baum | Global property investment: strategies, structures, decisions[END_REF].

Compared to unlisted funds, securitized real estate is sought for its liquidity, an attribute involving "homogeneous, predictable and standardized features that enable financial actors to convert [an asset] into cash quickly and easily" (Gotham, 2009: 357). Although liquidity exposes REIT shareholders to stock market volatility [START_REF] Lizieri | Real estate investment in global financial centers: risk, return and contagion[END_REF], it allows for the flexible management of mixed-asset financial portfolios, which helps optimize capital allocation and reduce the risk of capital immobility [START_REF] Corpataux | Increased Capital Mobility/Liquidity and its Repercussions at Regional Level Some Lessons from the Experiences of Switzerland and the United Kingdom (1975-2000)[END_REF]. Securitization also expands the scope of the real estate "investable universe" by contributing to the interconnection of financial circuits through which investors can gain cross-border, crosssector and cross-cycle diversification benefits (Aveline-Dubach, forthcoming).

The increasing entanglement of finance and the built environment makes cities increasingly vulnerable to the movements of global finance, and in return multiplies the systemic risks initiated by real estate crises [START_REF] Halbert | The Financialization of Business Property and What It Means for Cities and Regions[END_REF]. Residential real estate, in particular, has proved to be the main catalyst of such recent crises [START_REF] Crowe | How to deal with real estate booms: Lessons from country experiences[END_REF]. And yet, this has not prevented the development of global real estate investment networks -and corresponding forms of securitization -in a growing number of countries, with the noticeable exception of China. While in 1994 only four countries had REIT markets, today REITs are established in 37 countries (through 849 entities)3 and account for more than half of the real estate securities markets globally (Baker & Chinloy, 2014 : 254). Such a growth would not have been possible without the support of States in two key ways: i) by enacting regulations in support of REIT investment, and; ii) by adapting urban planning systems to the needs of financial investors.

Real estate securitization and the State

First and foremost, the adoption of REIT regimes requires national States to establish regulatory, tax and legal frameworks. The rules vary between countries, but common principles define how REIT operate as investment vehicles. A key aspect is their advantageous tax status [START_REF] Baker | Public real estate markets and investments[END_REF]. In most jurisdictions, REITs are exempted from corporate income tax if they redistribute most of their profits to their shareholders (typically 85-90 percent). While shareholders are subject to taxation on their dividends and the sale of their shares, local tax legislation often provides alternative mechanisms to reduce this burden [START_REF] Liu | Real estate investment trusts: Performance, recent findings, and future directions[END_REF]. In particular, shareholders domiciled in foreign countries -especially tax havenscan enjoy particularly favorable tax conditions depending on the agreements signed between different jurisdictions. For this reason, REITs offer higher returns than shares of publicly traded real estate companies (Newell, 2012). This substantial taxpayer support for REIT investment is generally not subject to public debate, since REITs are presented as active contributors to local economic growth and urban regeneration by providing key elements of corporate investment [START_REF] Henneberry | Calculated inequality? Portfolio benchmarking and regional office property investment in the UK[END_REF] through their large-scale portfolios of stabilized, long-term commercial properties [START_REF] Boisnier | Les Sociétés Foncières: entre finance et ville durable[END_REF].

Governments also impose financial regulations for assessing REIT real estate performance and asset management, which have the effect of spreading financial standards and practices well beyond REIT markets [START_REF]New Patterns of Property Investment in "Post-Bubble" Tokyo: The Shift from Land to Real Estate as a Financial Asset[END_REF]. This is particularly problematic in the residential sector, as housing becomes seen as a financial commodity rather than as the basis for household stability and security [START_REF] Marcuse | In defense of housing: The politics of crisis[END_REF]. [START_REF] Jones | Private investment in rented housing and the role of REITS[END_REF], following excessive deregulation of the banking industry [START_REF] Renaud | The 1985 to 1994 Global Real Estate Cycle: An Overview[END_REF]. A decade later, Japanese REITs emerged from the bursting of the "land bubble", also triggered by a deregulated environment for bank credit and urban development (Aveline-Dubach, 2014). More recently, the global financial crisis has also been a major catalyst for the current expansion of REITs. In the wake of this event, the large-scale devaluation of property markets generated vast inventories of distressed housing properties that financial investors were particularly well-placed to acquire. In Ireland, Spain and Greece, a significant number of housing units were securitized and pooled into REIT portfolios [START_REF] Chilton | The impact of single-family rental REITs on regional housing markets: A case study of Nashville[END_REF][START_REF] Waldron | Capitalizing on the State: The political economy of Real Estate Investment Trusts and the 'Resolution'of the crisis[END_REF][START_REF] Alexandri | Who loses and who wins in a housing crisis? Lessons from Spain and Greece for a nuanced understanding of dispossession[END_REF], while a new class of "singlefamily rental REITs" emerged in the USA -alongside the traditional multifamily model -from the purchase of distressed homes at foreclosure auctions [START_REF] Chilton | The impact of single-family rental REITs on regional housing markets: A case study of Nashville[END_REF][START_REF] Alexandri | Who loses and who wins in a housing crisis? Lessons from Spain and Greece for a nuanced understanding of dispossession[END_REF].

In the case of Ireland, [START_REF] Waldron | Capitalizing on the State: The political economy of Real Estate Investment Trusts and the 'Resolution'of the crisis[END_REF] highlighted the importance of REITs in reducing the debt burden of the country's failing banking sector and attracting capital to the distressed property market, coordinated by what the author called a "financial chain" mobilized by policymakers to reconnect the struggling real estate market to global financial investors.

Beyond this post-crisis function, REITs offer State authorities ready-to-use solutions to release locked-up capital in publicly owned assets through en-bloc sales. The structural deterioration of government budgets and incremental "colonization" [START_REF] Chiapello | La financiarisation des politiques publiques[END_REF] of State policies by financial logics, instruments and practices have led to the privatization of entire public housing estates [START_REF] Marcuse | In defense of housing: The politics of crisis[END_REF]. A portion of these have been incorporated into REIT platforms [START_REF] Wijburg | The alternative financialization of the German housing market[END_REF][START_REF] Bernt | Privatization, financialization and state restructuring in eastern Germany: the case of Am Südpark[END_REF][START_REF] August | Gentrification, suburban decline, and the financialization of multi-family rental housing: The case of Toronto[END_REF] as part of a process described as "financialized privatization" [START_REF] Aalbers | The financialization of housing: A political economy approach[END_REF]. The expansion of securitized real estate has involved other categories of publicly-owned facilities such as hospitals, prisons, or universities [START_REF] Wijburg | The differential impact of real estate financialization on the political economies of Germany and France[END_REF]. In France, this process has unfolded even within the State, with the creation of a REIT structure -Icade, currently the fifth largest French REIT-by a para-state banking institution [START_REF] Fretigny | Financer la cité: la Caisse des dépôts et l'économie politique du développement urbain[END_REF][START_REF] Wijburg | Reasserting state power by remaking markets? The introduction of real estate investment trusts in France and its implications for state-finance relations in the Greater Paris region[END_REF].

However, investment opportunities are carefully examined by REITs, which place a high importance on the location of the properties. REITs' building portfolios tend to be hyperconcentrated in a small number of cities -primarily financial centers -in well-connected central or neighboring areas (Aveline-Dubach, 2016). Cognitive biases and benchmarking practices inherent in financial investment contribute to this spatial selectivity (Henneberry & Mouzakis, 2014). Yet it is influenced above all by the liquidity risk carried by securitized vehicles. As publicly-traded entities, REITs are exposed to shareholders' "permanent exit capacity" (i.e. possibility to sell their shares at any time [START_REF] Crevoisier | Les territoires de l'industrie financière: quelles suites à la crise de 2008-2009?[END_REF] in addition to systemic risks. To ensure stable income streams, portfolios are composed of "liquid" properties (i.e. with the maximum re-sale potential). Such assets are sought in the prime areas of each market, targeting buildings that have a stable appreciation rate and low vacancy risk, and which are preferably occupied by major firms with high long-term leases [START_REF] Boisnier | Les Sociétés Foncières: entre finance et ville durable[END_REF]. Opportunities to acquire such high-profile properties tend to occur through the sale-leaseback of corporate real estate, a process driven by shareholder value [START_REF] Rutherford | Empirical Evidence on Shareholder Value and the Sale-Leaseback of Corporate Real Estate[END_REF] and occasionally encouraged by State policies [START_REF] Boisnier | Les Sociétés Foncières: entre finance et ville durable[END_REF]. Importantly, this effort to identify and acquire suitable buildings lead many REITs to become active promoters of urban change according to their own standards.

Theorists have drawn attention to the growing dominance of financial actors in urban decision-making processes. Several have demonstrated how financial investors interfere in city development -generally via locally-embedded developers -by co-defining the nature of urban projects in terms of their intended purpose (e.g. multifunctional buildings to mitigate vacancy risks), size (e.g. minimum investment thresholds for economies of scale) and occupation (e.g. large-size firms) [START_REF] Guironnet | Building cities on financial assets: The financialisation of property markets and its implications for city governments in the Paris cityregion[END_REF][START_REF] Theurillat | Sustainability and the anchoring of capital: negotiations surrounding two major urban projects in Switzerland[END_REF].

The influence of institutional investors in the built environment is also facilitated by real estate-financial lobbies that seek to align planning policies with investor interests. A nascent body of literature has turned attention to the informal strategies and tactics mobilized by these actors. Waldron showed how an epistemic community of financial and development actors used the narrative of "financial viability" in post-crash Ireland to radically deregulate the local planning system. Under the pretext of freeing up barriers to residential supply, the Irish government adopted an impressive package of reforms covering "virtually all of the development sector's policy prescriptions" (Waldron, 2019: 700). Similar conditions are observed in the United Kingdom, where [START_REF] Slater | The housing crisis and liberal Britain, free market think thanks and the production of ignorance[END_REF] and [START_REF] Haughton | Think tanks and the pressures for planning reform in England[END_REF] give account on the successful strategies of free market think tanks, business groups and professional organizations to make planning and housing policies more market-oriented and hostile to regulatory action against home price inflation on the grounds that it impedes market equilibrium. Finally, Kutz (2016) [START_REF] Kutz | Where to put the spare cash?" Subprime urbanization and the geographies of the financial crisis in the Global South[END_REF] and [START_REF] Krijnen | The urban transformation of Beirut: An investigation into the movement of capital[END_REF]

The housing sector as a new frontier for securitized real estate

Despite the gradual widening of their sectoral scope, REITs have for long neglected the residential housing market. Back in the mid-2000s, residential assets were used at best as a complement to commercial property in REITs' mixed-use portfolios. The United States stood out as an exception with specialized trusts in the residential sector established in the early 1990s, and which recorded significant growth [START_REF] Jones | Private investment in rented housing and the role of REITS[END_REF]. In France, when some large private property groups transformed into REIT structures in the early 2000s, their residential portfolios were almost entirely divested (Nappi-Choulet, 2012).

The weak attractiveness of the housing private rental market was due to a range of demandand supply-side constraints, which led to low returns compared to commercial uses (for example, 8% versus 19% in 2005 in Britain). 4 On the demand side, the scope of the private rental sector was reduced in the post-war period, being sandwiched between social rental housing and owner-occupation. In a majority of countries, including post-socialist economies, home-ownership was heavily promoted to middle income households, as part of a "social package" associated with life achievement [START_REF] Rowlands | Young peoples? Perceptions of housing tenure: a case study in the socialization of tenure prejudice[END_REF]. This was made possible by the unprecedented expansion of mortgage markets, alongside strong State support, often in the form of subsidized loans, subsidized homeowner saving schemes, and favorable tax regimes [START_REF] Stephens | Globalisation and Housing Finance Systems in Advanced and Transition Economies[END_REF]. Ronald stressed the ideological dimension conveyed by the preference for this type of tenure, especially in English-speaking countries where "the owner-occupier has been elevated as a better type of citizen, neighbor and even parent" (Ronald, 2008b: 2). In East Asian developmental States, the strong emphasis on ownership was even more acute where mass housing construction was made a pillar for economic growth [START_REF] Oizumi | Transformations in housing construction and finance[END_REF][START_REF] Ronald | Comparing Homeowner Societies: Can we Construct an East-West Model?[END_REF][START_REF] Wu | Planning for growth: urban and regional planning in China[END_REF] and housing a "store of welfare resources that can be carried across generations or be exchanged" (Ronald, 2007: 477). China represents the extreme culmination of this ideology of homeownership where some 52 million empty housing units are held by households (Aveline-Dubach, 2020). In countries that show strong support for home-ownership, socio-ideological forces and cultural prejudice have led to the stigmatization of rental housing as a residual and poorer form of tenure [START_REF] Rowlands | Young peoples? Perceptions of housing tenure: a case study in the socialization of tenure prejudice[END_REF].

The poor quality of the stock in a number of private rental markets has contributed to maintaining the negative perception of this sector. In most countries, the rental supply has been traditionally dominated by small-scale landlords (individuals, companies, employers, non-profit organizations) pursuing highly diverse economic objectives [START_REF] Crook | Private rental housing: Comparative perspectives[END_REF].

Due to the fragmentation of their interests, residential landlords have not gained political influence [START_REF] Gilbert | Rental housing: The international experience[END_REF] Regulations protecting tenants' rights in many jurisdictions are regarded as oppressive and risky [START_REF] Scanlon | Towards a sustainable private rented sector: The lessons from other countries[END_REF].

However, the private rental market has undergone structural change over the two past decades, to the point of becoming a new frontier for finance capital [START_REF] Rolnik | Late neoliberalism: the financialization of homeownership and housing rights[END_REF]. On the demand-side, the market's size has expanded due to three key drivers. First, the worldwide wave of housing price inflation in metropolitan regions has produced a "global urban housing affordability crisis" [START_REF] Wetzstein | The global urban housing affordability crisis[END_REF] [START_REF] Scanlon | Towards a sustainable private rented sector: The lessons from other countries[END_REF]. Third, the private rental sector has become increasingly attractive to a wide range of households that place a strong emphasis on the lifestyles of inner-city locations. From young workers to higher-income tenants, some dwellers choose to live in the vibrant, if unaffordable, downtown neighborhoods near their place of work, while others prefer to save for a future home purchase while living in less expensive areas [START_REF] Hulse | A private rental sector paradox: unpacking the effects of urban restructuring on housing market dynamics[END_REF].

The revival of the private rental sector is nevertheless due, to a large extent, to the growing concern of public authorities for the rental market. State initiatives have mainly focused on supply-side policies, aimed at creating a favorable climate for financial investment in rented housing. This is evident not only in the widespread deregulation of rental protection and the privatization of public housing estates, but it also shows up in the broader shift in the liberalization of planning policies, regulations and redevelopment projects, contributing to increasingly densely populated urban areas [START_REF] August | Gentrification, suburban decline, and the financialization of multi-family rental housing: The case of Toronto[END_REF]. Among the industrialized countries, few cities have experienced such a dramatic rescaling of their urban fabric as Tokyo has in the past decades. The next section of the paper will document how REITs are taking part in this process.

The Growth of Japanese Residential REITs in the Post-Bubble Era

Establishing REIT channels to revitalize the struggling economy

The establishment of the J-REIT regime in the early 2000s took place in the context of a sudden change in Japan's urbanization pattern after the collapse of the previous economic bubble.

The severe recession ended the period of steady GDP growth that had underpinned the suburbanization of home-ownership. Tokyo had become a financial and commercial hub in the 1980s through of the internationalization of Japan's productive systems and the development of the financial sector, and aspired to evolve into a major economic center in Asia [START_REF] Machimura | The Urban Restructuring Process in Tokyo in the 1980s: Transforming Tokyo into a World City[END_REF]. The triggering of the bank debacle not only put an end to this ambition, paving the way for the rise of Hong Kong and Singapore as regional financial hubs, but it also opened up a long phase of deflation known as the "lost decade", then later "the lost 20 years". At the same time, Japanese society became aware of the extent of its ageing population, although this had already been underway since 1973. In 1990, the fertility ratio fell to 1.54 children per woman, below the level of the "Year of Hinoeuma" (1966) when births were postponed due to superstitious beliefs. This observation came as a shock by revealing the structural erosion of the Japanese fertility that had been healthy for nearly two decades. It was therefore seen as inevitable that the Japanese population would experience negative growth, which was later confirmed in 2008.

Faced with these major economic and demographic challenges, after a few years of delay the Japanese government undertook a series of measures to create the conditions for a new wave of capital growth and investment in Tokyo's built environment. This began with the recognition that Tokyo was a world city and the country's only economic engine, and as such, it should be supported to promote wider national economic development (Child Hill & Kim, 2000;Leary & McCarthy, 2013;[START_REF] Buhnik | The dynamics of urban degrowth in Japanese metropolitan areas: what are the outcomes of urban recentralisation strategies?[END_REF]. The consensus was that urban regeneration in particular was needed to enhance Tokyo's competitiveness against emerging Asian rivals [START_REF] Tsukamoto | Urban regeneration and neoliberal state reform, changing roles of cities in the Japanese developmental state[END_REF]. In the mid-1990s, the Metropolitan Government changed Tokyo's development vision from being a "city of the world" to that of a "city friendly to residents" [START_REF] Kamo | An aftermath of globalisation? East Asian economic turmoil and Japanese cities adrift[END_REF] and thereby undertook a major campaign to "bring home and work closer together" [START_REF] Ito | Information on Tokyo's regeneration[END_REF]. The official aim of this policy was ostensibly to address the adverse effects of the mono-functional office land use in central Tokyo, but the main concern was actually to revitalize the city's struggling land market by encouraging real estate diversification (Aveline-Dubach, 2014).

Indeed, it was of the utmost importance to ensure the stabilization of land values in Tokyo, particularly in business districts where prices in some places had fallen by 70% from their peak level. As the banking sector was struggling with underperforming loans that were largely collateralized by undervalued land assets, large-scale mixed-used urban redevelopment projects were regarded as crucial to saving the Japanese financial system and restoring confidence in the nation's economy [START_REF] Fujita | Financial crises, Japan's state regime shift, and Tokyo's urban policy[END_REF][START_REF] Tsukamoto | Neoliberalization of the developmental state: Tokyo's bottom-up politics and state rescaling in Japan[END_REF]. The Ministry of Construction, which is continuously under the influence of real estate conglomerates and related lobbies, took a series of incremental measures to increase building density and decrease the time for administrative approval of development permits. The concept of "effective land use" (tochi yûkô katsuyô) was applied as a key principle in urban planning policies in order to directly increase land values through higher building densities.

As documented by Sorensen and colleagues [START_REF] Sorensen | Urban renaissance as intensification: Building regulation and the rescaling of place governance in Tokyo's high-rise manshon boom[END_REF][START_REF] Sorensen | Uneven processes of institutional change: path dependence, scale and the contested regulation of urban development in Japan[END_REF], these initiatives started between 1995 and 1997 with a change in the calculation mode of floor-area ratios (FARs) and the building envelopes previously in force. This was followed in 1999 by the privatization of the process to examine building permits. The deregulation of FARs took a further, more significant, turn under the Koizumi administration (2001)(2002)(2003)(2004)(2005)(2006), with the creation of a Regeneration Office under the direct supervision of the Prime Minister. This "Headquarters for the Rejuvenation of Cities", was given the task of designating special areas for the construction of high-rise buildings in major metropolitan regions, particularly the capital. Tokyo's central wards saw the rapid emergence of densely built large-scale urban redevelopment areas where the administrative procedures and authorizations for construction were outsourced to private firms. The densification policy was made all the more effective by the election of Governor Ishihara at the head of the Tokyo Metropolitan Government (1999Government ( -2012)), who actively supported the physical restructuring of Tokyo's center and waterfront promoted by the property conglomerates [START_REF] Waley | Pencilling Tokyo into the map of neoliberal urbanism[END_REF][START_REF] Saito | Shifts in Tokyo's world city status and the urban planning response[END_REF]. High-rise residential towers of 30 to 50 stories thus became familiar in an urban fabric once dominated by 3 or 4-storey buildings [START_REF] Sorensen | Urban renaissance as intensification: Building regulation and the rescaling of place governance in Tokyo's high-rise manshon boom[END_REF]. In Eastern Tokyo, land use policies allowed for the sudden conversion of industrial land into residential use, which gave rise to an impressive growth of densely populated high-rise residential blocks. This spectacular re-centering of building construction reinforced the effects of demographic decline, and aggravated urban shrinkage and housing vacancy on Tokyo's urban margins [START_REF] Phelps | Edge City Denied? The Rise and Fall of Tokyo's Outer Suburban "Business Core Cities[END_REF][START_REF] Kubo | Aging suburbs and increasing vacant houses in Japan[END_REF], 2019).

The return of residential functions to Tokyo's central areas was accompanied by the expansion of new neighborhood retail stores and other local services. Condominium-style housing, which had only been a first step to homeownership and suburban single-family residential living [START_REF] Ronald | Housing commodities, context and meaning: Transformations in Japan's urban condominium sector[END_REF], became increasingly appreciated for its convenience and ability to respond to changes in household structures and lifestyles.

Yet these large-scale urban projects had to be financed at a time when the banking sector, plagued by an accumulation of underperforming loans, could not meet the demand for real estate credit. Yet, the drop in land prices was nevertheless likely to boost real estate returns.

In 1997, the establishment of special purpose vehicles in real estate was approved by the government, and unlisted funds backed by American investment banks began to operate in Tokyo's property markets. Japanese real estate groups soon followed suit, acquiring the skills of real estate asset management through partnerships with US-based financial firms. When REIT channels were established on the Tokyo stock exchange in 2001, domestic real estate conglomerates used these platforms to transfer part of their properties while keeping control over J-REITs' asset management structures. This allowed them to realize immediate gains, scale down their balance sheets, and redeploy capital into redevelopment projects [START_REF] Ooi | The growth of REIT markets in Asia[END_REF]. In this process, the residential housing market enjoyed steady growth by benefitting from the deregulation of rental agreements in 1999 [START_REF] Hirayama | The role of home ownership in Japan's aged society[END_REF] and suffering from less turbulence than other sectors during the Global Financial Crisis.

The rise of rental condominiums during profound economic and demographic crisis

The sustained expansion of residential REITs reflects the confidence of asset managers and investors in the growth potential of the Tokyo's private rental sector. The demand for rental housing in Tokyo is driven by the same factors as found in other metropolitan areas of postindustrialized economies, but Japan's path to a finance-led growth regime and acute demographic transition are distinctive in this regard. Home-ownership has become a challenge for the younger generation despite the significant decline in Tokyo's housing prices since 1990. The purchasing power of first-time buyers has been considerably weakened by the decline of State support (the dissolution of the Government Housing Loan Corporation in 2007) and changing employment conditions. According to [START_REF] Yamada | How has the Japanese mode of regulation changed? The whereabouts of companyism[END_REF], workers previously enjoyed a stable working environment through two major arrangements:

employment security and company management security. The first, which concerned essentially regular male employees of large corporations, was ensured by a compromise whereby workers committed themselves to loyalty and dedication to their company, in exchange for lifetime employment and seniority systems for wages and promotions. The second was known as the "main banking system", whereby companies formed privileged partnerships and cross-shareholdings with banks, which in turn granted them various conditions for credit relief in the event of economic difficulties. The liberalization of financial markets radically altered these two arrangements. Large companies have come to rely on market-based financing. As they have become more vulnerable to economic downturns, they adapted their workforces to business changes by limiting job security to carefully-selected employees [START_REF] Yamada | How has the Japanese mode of regulation changed? The whereabouts of companyism[END_REF]. The dismantling of corporate security also included the gradual disappearance of corporate welfare housing. Large corporations used to own lowcost housing units rented to their employees, families and promising young staff -the socalled "golden eggs"-to increase their productivity [START_REF] Tanaka | Change in management environment and the transition of company housing[END_REF]. In 1993, this type of accommodation accounted for 6.4% of the housing stock in the Greater Tokyo area, almost as much as public housing (7.1%). Two decades later, the share of company housing dropped to 2.3%. 5 Corporate restructuring forced companies to sell these "non-core 5 Statistics of the Ministry of General Affairs, https://www.stat.go.jp/.

assets" on a massive scale, driving their young regular employees into the private rental market.

The growth of the private rental sector has also been highly influenced by demographic change. Declining birth rates and late marriages have led to an increase in the number of small households (singles and couples), undermining the standard family model that was the backbone of home-ownership [START_REF] Ronald | Home alone: the individualization of young, urban Japanese singles[END_REF]). Declining population rates further discouraged young households from investing in housing as a store of value for old age. Although the Greater Tokyo region is still experiencing a positive population growth rate (0,47 percent from 2015 to 2019 according to United Nations' estimates), which is expected to remain positive until 2020-2025, housing prices will inevitably deteriorate as the current younger generation reaches retirement age. As a result, the proportion of owner-occupiers under 40 has regressed to 34% of Tokyo's metropolitan area. 6 This leaves a large pool of potential tenants for buy-to-let investment, even though a fraction has limited income and unstable employment.

However, as in many other countries, the supply of private rental housing in Tokyo is typically provided by small-scale landlords, individuals or SMEs. For the latter, ownership of a residential building is mainly sought as collateral for bank loans. While no political agenda has been put in place to improve the quality of rental housing stock for this economic group, the sector has become an important avenue for diversification among Japanese property developers.

Of the 12 REIT entities surveyed, 10 are controlled by a property development group, which is itself often part of a larger conglomerate involved in trading, finance or railway transport (Mitsui, Itochu, Nomura, Tokyu, etc.). Under such schemes, the REIT entity can take advantage of the entire value chain of the sponsor group throughout the life-cycle of the properties. The REIT uses the so-called "pipeline" of the group, namely properties that are constructed within the group, according to REIT standards. 7 In general, the sponsor group manages the properties 6 Share of households whose main earner is under 40. Source Ministry of Land Affairs and Communication, "Housing and Land Survey of Japan", 2013. 7 The proportion of properties supplied by the sponsor group is nevertheless variable, ranging from 30% to 70%.

for a period of 6 to 10 years (often through an unlisted fund) before selling them to the REIT.

Once the buildings are acquired by the REIT, their maintenance is entrusted to a specialized subsidiary of the group, and when the properties become too old, they are often sold back to the sponsor. A significant portion of the REITs' properties were built during the years of the condominium boom (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008), which means that the average age of portfolio assets are between 10 and 15 years (Figure 2). This is considered to be quite old in Japan, where the traditional "scrap and build approach" (Ronald, 2008a) renders residential buildings that are more than 20-30 years old obsolescent. REIT properties are often disposed of after this term to avoid reducing portfolio performance with high building renewal costs.

Matching financial investors' expectations with highly risk-averse rental strategies

In addition to influencing the age of a property, asset managers develop risk-adjusted strategies to maximize their investment returns. In the current context of demographic decline, risk mitigation is primarily achieved by investing heavily in the Greater Tokyo region, the only part of Japan that is expected to experience continued population growth in the coming years. With few exceptions, residential J-REITs allocate approximately 80% of their portfolio value in this region. Other major cities such as Osaka and Nagoya still have a positive population balance, but they are expected to soon face declines. Yet, despite the risk of property value deterioration, asset managers choose to diversify investments in regional buildings because their higher yields help to boost portfolio performance.

Even within the Greater Tokyo region, the location strategies of asset acquisitions are very selective. Most REITs focus on the 23-ward area (see Figure 3), investing less than 12% of their portfolio value in large suburban cities such as Yokohama or Chiba. The eastern wards are very popular with REIT investment, because the release of large parcels of industrial land makes it easier to build higher buildings. The main target, however, remains Tokyo's five central wards (recently augmented by the Southern Shinagawa ward), where residential uses had virtually vanished during the bubble era, and which now receive more than 30% of the portfolio value.

Asset managers are looking for high entry barriers to take long-term positions in the most resilient sites of the country. Since property values are based on walking distance from major transit nodes, asset managers take a close look at the access times of the target buildings. For residences located in the inner-city, the walking time to the nearest subway or railway station is thus generally set at 5-7 minutes.

Another strategy to reduce risk is to limit the vacancy of dwellings by selecting highly solvent tenants. To this end, partnerships are forged with large companies (in sectors such as finance, IT and services) so as to generate a pool of "reliable" permanent renters. Agreements can be concluded directly with companies through long-term leases for one or more housing units, occasionally an entire building. Asset managers naturally prefer these contracts, but these only cover a quarter to a third of the rented units, as companies are reluctant to bear the vacancy risk. A large part of the agreements is concluded directly with the staff of the companies, and the remaining part with professionals (physicians, lawyers, etc.) who are insured in the event of default. Most of the companies' tenants are singles or DINKs ("dual income, no kids" couples) between the ages of 30 and 40. They are often graduates of major universities, and include a significant proportion of women. As the younger generation declines, competition between companies intensifies to capture these "golden eggs", hence the crucial importance of providing them with good housing conditions. Having sold most of their welfare housing, companies provide housing allowances to enable their staff to live in the inner city. The proximity of the home to work makes it possible both to meet the lifestyle expectations of these young employees and to make their workforce more profitable. allows for economies of scale in management. The second reason to favor employees of large corporations is the payment of a housing allowance. As REITs must ensure attractive returns for financial investors (cap rates currently from 3 to 5%), their rents are above the average level of the market, generally in the range of JPY 100,000 to 150,000 (USD 1,000 to 1,350) for a one-room condo. It follows that the housing allowance provided by the companies, often between JPY 40,000 and 50,000 (USD 360 to 450), is essential for many tenants to make ends meet. Finally, REITs benefit from the fact that young employees enjoy inner-city lifestyles, spend little time at home and are attracted to information technology connectivity. This significantly reduces the costs of ancillary space and service provisioning. For example, when local urban planning regulations do not require parking, only bicycle or two-wheel parking spaces are provided. Instead of guard services, the residences are equipped with large boxes for parcel delivery and a range of smart devices to ensure security such as dimple keys and security cameras-particularly popular with female tenants.

REITs further cut costs by achieving economies of scale in portfolio management. The design of the buildings is standardized and their maintenance is pooled within the portfolio. Inside the condominiums, the quality standards are lowered compared to similar housing units for owner-occupation, notably with thinner walls and doors, less sophisticated kitchens and bathroom equipment. It is important to note that the objective is not only to reduce costs, but also to mark the difference in housing tenure between renting home-ownership.

Despite the expected devaluation of property prices in the long-term, the ideology of homeownership is still prominent in Japan, and sponsor development groups maintain their priority on housing sales.

Owing to their effective profit maximization strategies, residential J-REITs have performed relatively well throughout their life, considering the difficult context in which they operate. In 2018, their annual return was 6.95% (as compared to a mere 0.01% for postal savings and 0.12% for Japan 10 Year government bond yield), breaking down into 4.87% rental income and 2.08% capital value 8 . Their capital is mainly held by domestic financial institutions looking for stable investment opportunities (Trust banks, regional banks, mutual funds) while foreign players, representing on average 10% of the capital, prefer to focus on the more profitable J-REIT office shares.

Conclusion

The experience of Japanese residential REITs converges with the findings of the literature on global corporate landlords which views State action as primordial. As in other jurisdictions where REIT regimes have been established, the Japanese State has helped to conceptually transform residential properties into liquid financial commodities and provided generous tax treatment to investors -mainly Japanese financial institutions. State policies at both national and local levels have combined to stimulate demand in Tokyo's real estate, thereby creating the conditions for an attractive rental housing market despite the country's acute demographic decline. This transition was achieved by: i) channeling the country's demographic and economic forces towards the capital region at the risk of devaluing other metropolitan areas; ii) intensifying construction in Tokyo's central wards through the massive liberalisation of planning and construction codes, even if it meant aggravating urban shrinkage and residential vacancy on the urban fringe, and ; iii) promoting the condominium as a new residential model (as part of the policy to "bring home and work closer together"), thus making the provision of rental condominiums more attractive to potential tenants and more rewarding for financial investors trough greater cash-flow streams.

While the State has been paramount to incentivize the REIT market, the social importance of property ownership has prevented the growth of demand in the private rental sector, especially the upper middle-class groups. Consequently, J-REITs' sponsor groups have forged arrangements with large domestic firms to host their employees. This has helped to maintain attractive returns of REITs' shares though above-market rents subsidized by company housing allowances. As the paper demonstrates, Japanese property conglomerates have seized residential REIT channels to create a lucrative fee-based business and to diversify their activities across the entire market's value chain, while strengthening their control over the sought-after areas of downtown Tokyo.

The arrangements between J-REITs and large corporations share commonalities with sale and lease-back strategies, insofar as the sale of corporate welfare housing assets is replaced by a leasing offer also intended for employees. Likewise, it is the same shareholder value logic that drives companies to divest their housing welfare assets so as to refocus on core business issues. Through the supply of REIT condominiums, employers can also better attract and take advantage of a highly skilled workforce by offering their promising young employees accommodation adapted to an urban lifestyle.

Although the "capture" of J-REITs' main residential supply by big employers is a serious means for attracting financial investment, it confines the trusts to a niche market despite the increasingly diversified needs for rental housing in Tokyo. Indeed, there is a profound mismatch between the mainstream demand in the private rental sector (emanating from nonpermanent employees, SME employees, senior citizens, foreigners, etc.) and REITs' provision of above-market condominiums with highly risk-averse criteria regarding tenant profiles.

Particularly problematic is that J-REITs focus on the provision of small housing units. While companies provided welfare housing for families, this form of housing is rarely supplied by J-REITs. In Tokyo, the rental market as a whole tends to specialize in more profitable small housing units [START_REF] Kubo | Diversification of the condominium supply in central Tokyo: Supply strategies of 'compact'condominiums[END_REF], which, according to [START_REF] Ronald | Home alone: the individualization of young, urban Japanese singles[END_REF], hinders family formation. From this perspective, residential J-REITs may actually exacerbate the declining birthrate trends in the capital region.

In conclusion, this paper provides an understanding of global landlords' practices from a new perspective, by highlighting their capacity to forge and expand their alliances with large corporations and domestic developers in a highly challenging economic and demographic environment. Moreover, by pointing to the mediation of large corporations in the private rental market, the paper brings these neglected actors into the framework of financialized rental housing, and puts the analysis into the broader context of employment. However, to widen our understanding of the outcomes of financialization, further analysis would be required regarding the practices of unlisted funds in Tokyo's rental condominium market, especially with reference to job-housing relations and family formation.

Figure 1 .

 1 Figure 1. Relationship between the sponsor group, the J-REIT and the State

Figure 3 .

 3 Figure 3. The location of residential REITs in the Tokyo Metropolitan Area

  [START_REF] August | Gentrification, suburban decline, and the financialization of multi-family rental housing: The case of Toronto[END_REF], the capitalization of specialized residential REITs in the United States was USD 180 billion. This figure does not include the value of residential REITs that are part of numerous diversified REITs.

1 As of February 2019. Source: https://j-reit.jp/en/market/, checked on March 16, 2019. 2 (source :https://www.reitnotes.com/reit-stocks-ranked-by-market-capitalization/2018-08-31, checked on March 16, 2019).

Bank(s) J-REIT Investors Asset manager Sponsor Group (and affiliated companies) Properties State agencies

	Paradoxically, REIT channels are often established to resolve major real estate crises arising
	from to State policies that have contributed to real estate investment. The first residential
	REITs were introduced in the United States after the collapse of the housing market in the
	early 1990s
	Invest	Build, renovate and manage properties
	Outsource
	Invest
	Pays dividends	Purchase/own
		Provide income
	Provide loan	(rental+capital value)
	Repay loan
	Exempt from corporate income tax	Increase allowable building density

  have examined the geographical spillover effects of financial crises on the planning and governance systems of cities beyond Europe. These and other studies demonstrate how the deregulation of capital

markets and corresponding expansion of domestic financial liquidity helped to intensify the rechannelling of financial investment into foreign housing stock, which directly impacted local government efforts to coordinate planning and development initiatives in cities of the Global South.

  , particularly since real estate lobbies centered the buyer's market on government agendas. There have nonetheless been many exceptions, notably in continental Europe, and in the United States where, according toJones (2017), the vast size of rented properties explains the stunning development of residential REITs. Clearly, financial investors seek economies of scale and do not favor small-size and spatially dispersed properties.

https://www.london.gov.uk/sites/default/files/gla_migrate_files_destination/current-issues-note-18.pdf, checked on February 2019.

  that has restricted access to home-ownership for previously solvent social groups, and aggravated social inequalities (Le Goix, Giraud, Cura, Le Corre, & Migozzi, 2019). Second, economic and social change in post-industrial economies has transformed housing needs. Greater employment flexibility/mobility, and the change in household formation (single-person and non-conventional households, rising marital age,

increasing divorces, population ageing, etc.) have entailed a gradual disengagement from home-ownership

https://www.ey.com/en_gl/real-estate-hospitality-construction/how-reit-regimes-are-doing-in-2018. Checked on March 1, 2019.

Kathryn Grant and Sandeep Sankoli, July 2017, Current Issues Note 18, "Institutional investment in housing", Greater London Authority, p. 8.

According to the AJPI annual return performance index for REIT investment (ARES https://jreit.jp/en/market/, checked on 10 November, 2019).


Sabri Allani

Richard Chbeir
email: rchbeir@acm.org

Taoufik Yeferny
email: yeferny.taoufik@gmail.comsadok.benyahia@fst.rnu.tn

Sadok Ben Yahia

Smart Directional Data Aggregation in VANETs

Keywords: VANET, Traffic Information System (TIS), Aggregation, Direction, Speed, Floating Car data (FCD)

The ultimate goal of a Traffic Information System (TIS) consists in properly informing vehicles about road traffic conditions in order to reduce traffic jams and consequently CO2 emission while increasing the user comfort. Therefore, the design of an efficient aggregation protocol that combines correlated traffic information like location, speed and direction known as Floating Car Data (FCD) is of paramount importance. In this paper, we introduce a new TIS data aggregation protocol called Smart Directional Data Aggregation (SDDA) able to decrease the network overload while obtaining high accurate information on traffic conditions for large road sections. To this end, we introduce three levels of messages filtering: (i) filtering all FCD messages before the aggregation process based on vehicle directions and road speed limitations, (ii) integrating a suppression technique in the phase of information gathering in order to eliminate the duplicate data, and (iii) aggregating the filtered FCD data and then disseminating it to other vehicles. The performed experiments show that the SDDA outperforms existing approaches in terms of effectiveness and efficiency.

I. INTRODUCTION

Vehicular Ad hoc Networks (VANETs) are a specific type of Mobile Ad hoc Networks (MANETs) [START_REF] Commission | Amendments of the commission's rules regarding dedicated short-range communication ser-vices in the 5.9 ghz band[END_REF] where each network node, representing mainly a vehicle, can communicate with other vehicles or an existing infrastructure, e.g., Road Side Units (RSU). Recently, there has been a rapid evolution of the technology required for supporting the VANETs. In fact, this technology has been adopted and supported by a lot of pioneering automobile manufacturers [START_REF] Commission | Amendments of the commission's rules regarding dedicated short-range communication ser-vices in the 5.9 ghz band[END_REF] (e.g. the US Department of Transportation (DoT), Toyota and Honda etc). This support has participated to the emergence of new ideas for VANET-based applications related to entertainment, safety and non safety information. Entertainment applications include multimedia streaming, P2P file sharing and Internet access [START_REF] Nadeem | Trafficview : Traffic data dissemination using car-to-car communication[END_REF]. Informational applications include all what concerns the notification of upcoming roadway hazards and traffic conditions, in addition to the collection and dissemination of weather information [START_REF] Nadeem | Trafficview : Traffic data dissemination using car-to-car communication[END_REF]. The safety applications have been the US DoT's prime focus efforts in vehicular networks including both assistance and collision warning [START_REF] Commission | Amendments of the commission's rules regarding dedicated short-range communication ser-vices in the 5.9 ghz band[END_REF]. Intelligent Transportation System (ITS) standards perform communication in VANETs so as to support transportation and cooperation services [START_REF] Commission | Amendments of the commission's rules regarding dedicated short-range communication ser-vices in the 5.9 ghz band[END_REF]. Advanced technologies (e.g., image and gesture recognition) have been incorporated to improve information exchanging and ITS effectiveness. As far as the development in the wireless communication field is generally concerned, the ITS applications are developed on the basis of car-tocar communication standards (i.e., Dedicated Short Range Communications (DSRC) and Wireless Access in Vehicular Environment (WAVE)) [START_REF] Commission | Amendments of the commission's rules regarding dedicated short-range communication ser-vices in the 5.9 ghz band[END_REF]. Also, the technology of Inter Vehicle Communication (IVC) is enabled for most applications, namely entertainment, enhanced driving, and active safety. Subsequently, it is a must to manage the available limitedbandwidth carefully and efficiently as it is a scarce resource. Thus, communication VANET protocols need a careful design. Hence, to cope with the limited bandwidth, the various requirements emerging from the applications themselves and the highly dynamic network topology. Particularly, the VANETbased applications requiring information gathering and aggregation (e.g., traffic information systems, weather information systems, parking spaces and travel time predictions) have to deal with the following challenges:

1) How to decide if two or more FCD messages must be aggregated or not? 2) How to select timely data to be aggregated (since not all data need to be collected after a certain time)? 3) How to consider data from far vehicles? 4) How to filter the unnecessary and duplicated FCD messages in order to avoid affecting the accuracy of the shared traffic information? 5) How to take into consideration the road traffic signals and speed limitations?

Several data aggregation techniques have been proposed in the literature [START_REF] Ibrahim | Cascade: Cluster-based accurate syntactic compression of aggregated data in vanets[END_REF][START_REF] Kumar | A framework for handling local broadcast storm using probabilistic data aggregation in vanet[END_REF][START_REF] Tsai | Aggregating data dissemination and discovery in vehicular ad hoc networks[END_REF][START_REF] Zhu | Delay-constrained data aggregation in vanets[END_REF]. However, in a high complex urban and highway network, the large amount of traffic information needs smart filtering and selection criteria after the aggregation process. However, all existing techniques mainly focus on combining the correlated items but none of them tries to use a suppression technique in order to eliminate the duplicated messages. To cope with these aforementioned requirements, we introduce here a new aggregation protocol called Smart Directional Data Aggregation (SDDA) that can deal proprely with traffic information in both urban and highway conditions. It consists in properly selecting relevant FCD messages that should be aggregated. To do so, SDDA provides three filters: (i) The first uses vehicle directions. In fact, each vehicle will aggregate only data that corresponds to its direction but only store, carry and forward uninteresting data. (ii) The second is done through road speed limitations, thus, if the average speed of the received FCD messages is higher than the maximum allowed speed then it will be ignored and replaced by the maximum allowed speed value. Doing so, we increase the aggregation accuracy. (iii) The last is performed is by integrating a suppression technique [START_REF] Wisitpongphan | Broadcast storm mitigation techniques in vehicular ad hoc networks[END_REF] in order to eliminate the duplicated inputs in the aggregation phase by providing a high accurate time slot based on the distance and the range that all vehicles should wait before rebroadcasting the FCD message. The rest of the paper is organized as follows. Section II describes the pioneering literature approaches to highlight their drawbacks regarding the aforementioned challenges. In Section III, we describe our proposed protocol for smart data aggregation. Section IV is dedicated to present the simulation settings as well as the evaluation of our suggested approach.

The last section concludes this paper and pins down several future directions.

II. RELATED WORK

In larger road networks, urban and highway, there is a huge amount of traffic information that quickly exceeds reasonable limits. In this respect, an appropriate aggregation mechanism can reduce the communication cost while obtaining useful aggregated information. Several related approaches have been proposed in the literature.

In [START_REF] Wischoff | Sotis -a self-organizing traffic information system[END_REF], the authors put forward Self-Organizing Traffic Informational Systems (SOTIS) where vehicles would periodically exchange their speed and position on the road. Each vehicle in SOTIS would calculate average speeds from neighboring vehicles before rebroadcasting the aggregated data. It would basically provide an outline of the current movement conditions through the use of periodic road information broadcasting. On the one hand, the main SOTIS limitation was related to the broadcast of the periodic Floating Car Data (FCD) since it did not ensure or even indicate how duplicated [FCD] messages that came from the same road segment could be aggregated together. On the other hand, the aggregation of the same duplicated FCD messages would reduce the accuracy of the final aggregated records.

Traffic View is another interesting approach described in [START_REF] Nadeem | Trafficview : Traffic data dissemination using car-to-car communication[END_REF]. It proposes a similar periodic approach based on broadcasting beacons that contained the FCD, like the average speed of a current road segment and its traffic density. The main difference between SOTIS [START_REF] Wischoff | Sotis -a self-organizing traffic information system[END_REF] and Traffic View was the computation of the average speed since the aggregation process in Traffic View was an accumulative of average speeds in the road (starting from the quickest vehicle to the slowest one). On the contrary, within SOTIS a vehicle in the center could aggregate all neighboring vehiculars' data within its range. It was blatant that the aggregated record in Traffic View consisted of just one time-stamp value, one position and one speed, as well as vehicular IDs. Hence, this would use better the bandwidth when transmitting messages to every individual vehicle.

CASCADE was suggested in [START_REF] Ibrahim | Cascade: Cluster-based accurate syntactic compression of aggregated data in vanets[END_REF][START_REF] Ibrahim | Data aggregation and dissemination in vehicular ad-hoc networks by data aggregation and dissemination in vehicular ad-hoc networks[END_REF] as an optimized Traffic View version [START_REF] Nadeem | Trafficview : Traffic data dissemination using car-to-car communication[END_REF]. It allowed compressing syntactic data in the aim of optimizing the use of a wireless channel and at the same time guaranteeing accurate aggregated information. Furthermore, it would divide a road into 12 rows (16m × 126m) of a cluster leading to a 1.5 km visibility (named a local view). Thus, when vehicles in a local view cluster shared their FCD messages with another cluster, they had an extended view of the whole road segment.

Tsai introduced in [START_REF] Tsai | Aggregating data dissemination and discovery in vehicular ad hoc networks[END_REF] a hybrid Aggregating Data Dissemination (ADD) approach that combined both the V2V and V2I models. This approaches also aggregate the number of available free parking spaces in a big region. To do that, the author split a map into a grid structure of square regions. The geodesic distance between these regions was in fact the Road Side Unit (RSU) communication range. The author defined four data aggregation levels in every region. In the first aggregation level, each vehicle would send its parking place, id, position and speed to the RSU center of the region. The RSU would aggregate all received data in the second level before rebroadcasting it to all vehicles in the region. In the third level, the vehicles in the extreme regions would share their traffic information, with the RSU sink. Finally, this later would aggregate, in the fourth level, all information coming from various regions before rebroadcasting it.

Kumar and Dave introduced in [START_REF] Kumar | A framework for handling local broadcast storm using probabilistic data aggregation in vanet[END_REF] a new multi-criteria decision-making for data aggregation. As a matter of fact, the proposed approach assisted a vehicle to decide about the relevance between data, for instance vehicle speed, vehicle direction and free parking space. Hence, the suggested system could decide if two or more input data were similar enough (syntactically or semantically) to be aggregated or not. To achieve this, the authors represented the knowledge base as a KD-tree data structure in order to check the relevance between nodes using the graph characteristics. Although interesting, the provided approach had major drawbacks but mainly it considered the location of aggregates and ignored all other properties (e.g., vehicle directions, maximum allowed speed, etc.). Indeed, this aggregation decision would only consider data coming from the same road segment.

Time aggregated graphs were introduced by George et al. in [START_REF] George | Spatio-temporal networks[END_REF]. They allowed formalizing the road networks and the spatio-temporal properties of the road as a graph data model which would generally support shortest-path query graph algorithms. These time-aggregated graphs could annotate the properties of edges and nodes with the intervals during the time of vehicle presence. However, to decide whatever multiple items could be aggregated or not, the spatio-temporal model was only based on the vehicle travel time, the event life time, and the location. Thus, the major drawback of such a model was that one could not exactly estimate the vehicle travel time, since it could change its direction at any time.

In the same vein, Zeki et al. introduced in [START_REF] Zekri | Building , sharing and exploiting spatio-temporal aggregates in vehicular networks[END_REF] an aggregation structure for events produced and exchanged in vehicular networks. More precisely, the proposed structure was based on a spatio-temporal model having two levels. The first one is a physical level consisting of a repository shared between all vehicles to share information without loss. The second one was a logical level where each driver would define his/her preferences for what information (s)he was interested in.. This model would manipulate the same shared knowledge base between all vehicles. The ability of being duplicate insensitive and being able to guarantee lossless exchanged information was a significant characteristic of their data structure. In addition, the storage space, mainly needed for the aggregation structure, was particularly limited to include a governable number of temporal dimensions. The main limitation of this solution was the maintenance and the privacy issues of such a shared knowledge base.

In [START_REF] Saleet | Intersection-based geographical routing protocol for vanets: A proposal and analysis[END_REF], the aggregation of the FCD was only based on the geographical characteristics of the area. For that purpose, the authors introduced the Region-based Location protocol Service Management Protocol (RLSMP) which aimed to reduce the updated positions as well as reducing the number of messages generated to locate car positions. Although the aggregation solution clearly reduced the network overload, it resulted in: i) more packet collisions and consequently more re-transmissions essentially due to the fact that the exchanged packets had a big size, and ii) longer delays owing to the processing carried out on the data.

To sum up, the aggregation process in the aforementioned approaches consists of three phases known as decision, fusion and dissemination:

1) The decision phase, where decision regarding the selection of data items to be fused is made 2) The fusion phase, which is related to the function of fusion. Therefore, all the similar data will be merged in one record 3) The dissemination phase, where aggregated data are broadcasted to other vehicles. Consequently, the existing aggregation schemes have several limitations, but mainly:

• Security: If the aggregation has the ability to decrease bandwidth consumption problems, it might make security issues harder to manage (e.g., the encryption and the decryption of multiple aggregated and compressed packets) [START_REF] Mohanty | Secure data aggregation in vehicular-adhoc networks: A survey[END_REF].

• Scalability: The existing schemes have medium aggregation time as well as low scalability. This is owing to the fact that when the number of the duplicated exchanged messages goes up, the number of collision problems rises as well [START_REF] Kumar | A framework for handling local broadcast storm using probabilistic data aggregation in vanet[END_REF]. • Genericity: Only few approaches [START_REF] Tsai | Aggregating data dissemination and discovery in vehicular ad hoc networks[END_REF][START_REF] Kumar | A framework for handling local broadcast storm using probabilistic data aggregation in vanet[END_REF] propose a generic model for both aggregation and dissemination mechanisms. In fact, such a combination and synchronisation is of paramount importance to avoid the broadcast storm problem [START_REF] Wisitpongphan | Broadcast storm mitigation techniques in vehicular ad hoc networks[END_REF] and to decrease the network overhead. • Filtering: The input items are not fully filtered out (many duplicated items and irrelevant items are not neglected) leading to a high level of network overload.

III. SMART DIRECTIONAL DATA AGGREGATION PROTOCOL Before describing our aggregation protocol, we present in the following subsection several definitions and concepts.

A. Preliminaries

To better understand our solution, we start by defining the main concepts used in our approach:

Definition 1: Lane (l): It is a one-way path with a paved surface that connects two spatial points on the map. Formally, l: Id, S, E, -→ SE, Speed min , Speed max , Status, Location where:

• Id: represents the identifier of the lane • S and E respectively represent the start and end points which the lane is connecting. Each point is represented by spacial coordinates (e.g., (x,y)) • Id: represents the road identifier • L: contains the set of lanes included in the road • Type: is used to indicate if it is a highway, urban, street, etc. • Name: is used to describe the road (e.g., street name) • Network Coverage: indicates the types of network covered within the road (e.g., GSM, WIFI, 3G, 4G, etc.) Definition 3: Vehicle (v): A vehicle is defined in our approach as follows: v: Id, Driving, Speed max , Positioning System, Brand, Type, Size, Environment, DSRC Range, Destination where:

• Id: represents the vehicle identifier • Driving: is the set of the driving settings (e.g., Preferred path, Deriving mode: Economic/sport, etc.) • Speed max : represents the vehicle maximum speed • Positioning System: refers to the geographical GISsystem used by the car (e.g., Google Maps, Open-StreetMaps, Bing Maps, etc.) • Brand: refers to the automobile manufacturer of the vehicle (e.g., BMW, Jeep, Toyota, etc.) • Type: is used to indicate the type of the car (e.g., Mini-Van, Sport, Light Truck, etc.) • Size: indicates the size of the vehicle • Environment: refers to the vehicle speed, weather, and the geographic location • DSRC Range: is the signal power of dedicated wireless short-range communication technology (e.g., 300m, 400m, etc.) • Destination: is the location to which a vehicle travels. A vehicle can perform three actions:

• Broadcast: It disseminates fcd messages using the suppression broadcasting technique defined in [START_REF] Wisitpongphan | Broadcast storm mitigation techniques in vehicular ad hoc networks[END_REF] • StoreCarryandForward: A vehicle can store, carry and then rebroadcast the same message using the rebroadcasting technique defined in [START_REF] Wisitpongphan | Broadcast storm mitigation techniques in vehicular ad hoc networks[END_REF] • CalculateAverageSpeed: It calculates the average speed using the aggregation function defined in section III-B • Receive: It receives all types of messages sent using the DSRC protocol [START_REF] Commission | Amendments of the commission's rules regarding dedicated short-range communication ser-vices in the 5.9 ghz band[END_REF] • CalculateDirection: A vehicle is able to determinate its direction using a positioning system (e.g., Google Maps) based on its location and its destination • LocateLane: A vehicle is able to locate the current driving lane based on its geographic location.

In our solution, we assume that our aggregation protocol runs on the top of the MAC-layer, so it requires no modification in IEEE 802.11p standard [START_REF] Commission | Amendments of the commission's rules regarding dedicated short-range communication ser-vices in the 5.9 ghz band[END_REF]. Also, we acquire that only one type of messages can be generated and sent, namely the Floating Car Data (FCD) known also as Floating Cellular Data [START_REF] Wischoff | Sotis -a self-organizing traffic information system[END_REF]. Definition 4: FCD message (f): The adopted FCD messages header structure is therefore defined as a 4-tuple: f: SenderId, SenderP osition, AverageSpeed, Destination where:

• SenderId: is the unique identifier of vehicle that sends the message • SenderP osition: contains the spatial position of the sender • AverageSpeed: is the average speed (computed using a function defined in Section III-B) • Destination: contains the future location of the vehicle sending the FCD message.

The size of the message is less than 2, 321 Bytes, which is the maximal allowed size as defined by 802.11p standard [START_REF] Commission | Amendments of the commission's rules regarding dedicated short-range communication ser-vices in the 5.9 ghz band[END_REF].

B. Aggregation protocol

Our aggregation algorithm deals with the following three scenarios: a unidirectional road, a bidirectional road, and an urban scenario. The aggregation function used in our protocol is the same average function used in SOTIS [START_REF] Wischoff | Sotis -a self-organizing traffic information system[END_REF]. We choose the main average speed aggregation function of SOTIS since our contribution can be seen as an optimisation of SOTIS. However, as mentioned in the related work, only three approaches, SOTIS [START_REF] Wischoff | Sotis -a self-organizing traffic information system[END_REF], TrafficView [START_REF] Nadeem | Trafficview : Traffic data dissemination using car-to-car communication[END_REF] and CASCADE [START_REF] Ibrahim | Data aggregation and dissemination in vehicular ad-hoc networks by data aggregation and dissemination in vehicular ad-hoc networks[END_REF], focus only on the aggregation of the vehicles speed without combining other driver preferences. The average speed aggregation function of SOTIS is defined as follows [START_REF] Wischoff | Sotis -a self-organizing traffic information system[END_REF]:

Vr,new = Vr,prev + Vr (1)
where Vr,new is the new average speed for the road r, Vr,prev is its previous average speed, and Vr stands for the average speed of the vehicles on the road r. Each vehicle has three aggregation cases: unidirectional-road, bidirectional-road and urban-city. Algorithm 1 depicts the behavior of a vehicle v upon receipt of an FCD message. 1) Unidirectional Road Case: On an unidirectional road, all vehicles drive in the same direction. Indeed, vehicles ahead must collect traffic information, aggregate and disseminate it to other vehicles located behind. Figure 1 depicts the unidirectional road scenario where lanes contain different speed limitations. Algorithm 2 contains the pseudo-code of processing this case. Briefly, when a vehicle v receives an FCD 1. If the received average speed is greater than the maximum allowed speed of the current road r, the vehicle receiving the message will keep its previous average speed Vr,prev and rebroadcast the value of the road maximum speed. Doing so, the propagated FCD message will warn other vehicles' drivers that ahead vehicles are driving very fast and exceed the maximum allowed speed. In Figure 1, only vehicles v 1 and v 2 will receive and deal with the FCD message sent form vehicle v 3 since they are in the broadcast range and drive behind it. However, vehicles v 1 and v 2 will ignore the received average speed, 140 km/h, and broadcast the road maximum allowed speed (of 130 km/h). Doing so, all the disseminated traffic information will follow the legal speed.

2) Bidirectional Road Case: On a bidirectional road, vehicles drive in two opposite directions. Indeed, vehicles in the opposite directions must collect traffic information, and then aggregate and disseminate it to other opposite vehicles as shown in Figure ??. The exchange of traffic information between the two opposite directions has a paramount importance. In essence, such an exchange would inform and warn all drivers about the traffic conditions ahead, which leads to avoid traffic jam and road accidents. Figure 2 illustrates the bidirectional road scenario where lanes contain different speed limitations. Indeed, vehicle v 4 will share its FCD average end if end if 20: end procedure speed (90 km/h) with vehicle v 1 , which will broadcast this information to other behind vehicles since the average speed is less than the maximum allowed speed. Algorithm 3 contains the pseudo-code of processing this case. In fact, the main difference between this case and the previous one is that in a bidirectional scenario, vehicles can accept FCD messages that come form the opposite side or vehicles ahead in the same lane and ignore other messages that come from behind. Actually, Fig. 2. Bidirectional road case the vehicles in the opposite side have a larger overview on the opposite traffic conditions, since they have been passed in front of it. Doing that allows guaranteeing that the FCD contains an aggregated value of the whole opposite lanes.

3) Urban Network Case: In urban network, vehicles drive in many different directions. Thus, vehicles can meet over cross roads and junctions. Indeed, they must exchange their traffic information ,as shown in Figure 3. However, using a blind aggregation method will affect the precision and the accuracy of the exchanged traffic information. This is owning to the fact that vehicles can share and aggregate the average speed of other vehicles that are not going to the same direction. Our aggregation model can solve this issue based on three filters: Algorithm 3 Aggregation algorithm on a bidirectional road with speed limitations if N OT Same(SenderDirection, V ehicleDirection) then end if 21: end procedure 1) A direction filter that ensures the aggregation of coherent FCD messages targeting the same road and direction

2) A suppression technique filter that ignores all duplicated FCD messages. In the current version of our protocol, we adopt the famous slotted 1-persistence broadcast protocol [START_REF] Wisitpongphan | Broadcast storm mitigation techniques in vehicular ad hoc networks[END_REF] 3) An aggregation filter that ignores all the received FCD messages that exceed the road maximum speed. In fact, if the average speed of any FCD message is greater than the maximum allowed speed, the received vehicle will ignore it and broadcast the maximum speed instead. In urban networks, when a vehicle gets an FCD message, it checks firstly the direction of the sender. Then if the received average speed is less than the road maximum speed limit, the vehicle will aggregate and disseminate the received FCD message. Otherwise, the received message will be ignored. Indeed, when a vehicle v receives a message, not in its Fig. 3. Urban scenario case direction, it will store, carry, and then forward it to other vehicles that may be going to this direction. Doing so, the traffic information will be disseminated to all vehicles in the road intersections. Algorithm 4 depicts a vehicle behavior upon receiving an FCD message in an urban situation.

IV. EXPERIMENTATIONS

In this section, we present the performance evaluation conducted to evaluate our aggregation protocol. We choose SOTIS [START_REF] Wischoff | Sotis -a self-organizing traffic information system[END_REF] and TrafficView [START_REF] Nadeem | Trafficview : Traffic data dissemination using car-to-car communication[END_REF] as baseline approaches since they focus only on the aggregated FCD without combining other data (like safety and non-safety events). CASCADE [START_REF] Ibrahim | Cascade: Cluster-based accurate syntactic compression of aggregated data in vanets[END_REF] will not be considered in our evaluation since it uses a compression algorithm on the top of TrafficView [START_REF] Nadeem | Trafficview : Traffic data dissemination using car-to-car communication[END_REF] to optimize the MAC-Layer utilization (this is out of scope of our study here but can also be applied by our approach after aggregation). We carry out our experiments using the Veins simulator framework 1 . Veins is an open source framework for Inter-Vehicular Communication (IVC) that combines both a road traffic micro-simulation model and an event-based network simulator. To conduct the experiments, we choose the default road map of the city of Manhattan in the framework. This city has a very high complex road topology that contains both urban and highway networks. We vary the amount of vehicles driving on the map from 200 to 1000, ranging from low traffic to high traffic.

1 http://veins.car2x.org/

A. Experimental settings

The road traffic simulation is performed by SUMO 2 while the network simulation is performed by OMNeT++ along with the physical layer modeling toolkit MiXiM 3 , allowing the employment of accurate models for radio interference, as well as shadowing the use of static and moving obstacles. With these two well-established simulators, nodes simulated by OMNeT++ 5.1.0 can interact with SUMO to simulate the influence of IVC on road traffic and mobility. We take advantage of these two simulators included in Veins to provide realistic models for 802.11p DSRC, PHY and MAC layers. The PHY and MAC parameters are defined according to the basic specifications of the 802.11p standard defined in [START_REF] Sommer | Bidirectionally coupled network and road traffic simulation for improved ivc analysis[END_REF]. In the MAC layer, we set the transmission power of a vehicle to 30mW to achieve approximately 300m of interference range

B. Evaluation metrics

The assessment of the performances of our protocol is carried out through the following metrics:

1) Overload: It stands for the total number of sent packets. Interestingly enough, the ultimate goal of any aggregation protocol is to avoid the overload problem [START_REF] Commission | Amendments of the commission's rules regarding dedicated short-range communication ser-vices in the 5.9 ghz band[END_REF] by looking for minimizing the number of messages exchanged in the network. The average overload is defined as follows:

Overload = sent packet vehicle (2)
2) Co2Emission: refers to the amount of Co2 getting out from vehicles and affecting the environment [START_REF] Cappiello | A statistical model of vehicle emissions and fuel consumption[END_REF]. Indeed, the ultimate goal of any traffic information system, and especially any aggregation protocol, is to reduce the total Co2 emmision [START_REF] Cappiello | A statistical model of vehicle emissions and fuel consumption[END_REF] by looking for minimizing as much as possible traffic jams and decreasing the active waiting time of vehicles in cross roads and on highways. The average Co2 emission is defined as follows:

Co2Emission = V ehicle Co2Emission vehicle (3)
3) Latency: It is the time needed to deliver the aggregated message to an interested vehicle. The average latency, AL, is defined as follows:

AL = (t vi -T ) Interested V ehicle (4)
where t i stands for the arrival time of the event message to a vehicle v i , and T is the time-stamp of the event. 4) FCD Duplication Ratio: It is the number of messages that are already sent and aggregated for a given message f . It is defined as follows: aggregation protocol is able to only aggregate an appropriate FCD message f for a given road r without duplication and to take into consideration the road maximum speed. Hence, the challenge will be to obtain precision values of the average speed propagated to vehicles compared to the real road traffic state. It is defined as follows:

Duplication ratio = Duplicated f f (5)
P recision(f ) = |IIV | |AIV | (6)
where IIV stands for the set of interested informed vehicles (i.e., only appropriate vehicles for a message f ), and AIV stands for the set of all informed vehicles that will aggregate the received FCD message f (i.e., interested as well as not interested vehicles for an FCD f ). The average precision is defined as follows:

AverageP recision = P recision(f ) f (7)

C. Results

Figure 4 shows the evolution of the overload and co2emission values in different density networks. As expected, using our aggregation protocol, the overload level is decreased (cf. Figure 4-a). This is owed to the fact that the number of duplicated messages is eliminated and the propagation of unnecessary aggregated FCD messages to uninterested vehicles is reduced. As a result, our protocol SDDA enables keeping a low overload. Moreover, Figure 4-b shows that SDDA has a low Co2 Emmision value in different network densities. It is worth mentioning that our protocol decreases the co2Emmision value in comparison to SOTIS and TrafficView by almost 70% since vehicles will have a precise traffic information and will avoid traffic jams. Figure 4 also demonstrates that the overload and co2Emmision values will rise for all protocols when the number of vehicles increases. This interesting performance is related to the fact that our strategy permits ignoring all the unnecessary FCD messages when vehicles move based on direction and speed limitation filers. Moreover, Figure 5b indicates that the number of duplicated messages in our proposed solution is 99% less than other solutions thanks to the usage of the Slotted-1 persistence suppression technique [START_REF] Wisitpongphan | Broadcast storm mitigation techniques in vehicular ad hoc networks[END_REF] being able to eliminate all the duplicated messages. Finally, Figure 6-a depicts that the latency of our SDDA strategy is slightly less than that of other strategies. This is due to the high overload level of SOTIS and TrafficView which increase the network collisions that affects the latency time. As expected, the packet loss ratio in Figure 6-b is inversely proportional to the traffic density. In fact, it decreases as far as the number of vehicles grows. This can simply be explained by the fact that when raising the number of vehicles in the network, the communication overload as well as the message reception errors go up, which leads to increase the ratio of lost packets. Consequently, this fact will undoubtedly increase the number of vehicles that will not receive the messages or will receive damaged packets (unreadable, corrupted, etc.). To sum up, the simulation results highlight that our strategy performs much better than the baseline strategies from different perspectives (overload, co2emission, aggregation precision, packet loss and latency). Also, it is important to note that our strategy is genetic and can be adapted to any other aggregation context like road conditions, commercial advertisements and trip travel time.

V. CONCLUSION

In this paper, we have introduced SDDA as a generic smart directional data aggregation model protocol to exchange information about traffic in a VANET to overcome several limitations related to existing approaches. The main thrust of our protocol stands for an adequate targeting of using direction and road speed limitation. Doing so has allowed meeting several goals, namely reaching a low overload ratio as well as a high aggregation precision. Extensive experimental work has shown that SDDA obtains very interesting results in comparison with those provided by pioneering approaches of the literature. Avenues of future work are as follows:1)We are actually working on real-word validation scenarios and we plan to extend the model to deal with other aggregation issues in VANET like parking spaces, road conditions, and trip travel time, 2) Provide a generic compression algorithm for FCD messages in order to reduce the bandwidth usage, and 3) Integrate other dissemination protocols in our aggregation model in order to provide a more generic aggregation and dissemination protocol.

Fig. 1 .

 1 Fig. 1. Unidirectional Road Case message, it checks if the message comes from a farther sender on the same road or not. If yes, it computes the new updated average speed Vr,new based on Equation1. If the received average speed is greater than the maximum allowed speed of the current road r, the vehicle receiving the message will keep its previous average speed Vr,prev and rebroadcast the value of the road maximum speed. Doing so, the propagated FCD message will warn other vehicles' drivers that ahead vehicles are driving very fast and exceed the maximum allowed speed. In Figure1, only vehicles v 1 and v 2 will receive and deal with the FCD message sent form vehicle v 3 since they are in the broadcast range and drive behind it. However, vehicles v 1 and v 2 will ignore the received average speed, 140 km/h, and broadcast the road maximum allowed speed (of 130 km/h). Doing so, all the disseminated traffic information will follow the legal speed.2) Bidirectional Road Case: On a bidirectional road, vehicles drive in two opposite directions. Indeed, vehicles in the opposite directions must collect traffic information, and then aggregate and disseminate it to other opposite vehicles as shown in Figure??. The exchange of traffic information between the two opposite directions has a paramount importance. In essence, such an exchange would inform and warn all drivers about the traffic conditions ahead, which leads to avoid traffic jam and road accidents. Figure2illustrates the bidirectional road scenario where lanes contain different speed limitations. Indeed, vehicle v 4 will share its FCD average

4

 4 Aggregation algorithm in urban network 1: P ← v.Environment.Location 2: VD ← v.Destination 3: VDirection ← v.CalculateDirection(P,VD) 4: VSpeed ← v.Environment.Speed 5: LaneSpeedM ax ← v.LocateLane().SpeedMax 6: procedure VEHICLE IN URBAN CITY(f cd)

2Fig. 4 .Fig. 5 .Fig. 6 . 5 )

 4565 Fig. 4. Variation of average overload/Co2 emission values w.r.t. number of vehicles

  Speed min and Speed max represent the minimum and maximum speed limits , respectively • Status: indicates the lane situation (e.g., closed, open, restricted, etc.) • Location: is the geographical coordinates of the lane in the map. Definition 2: Road (r): It contains at least one or several lanes having the same or different directions. Formally, r: Id, L, Type, Name, Network Coverage where:

	•	-→ SE: represents the directed vector segment from the point
		S to E
	•