## 1 Introduction  
In the last few decades, Intelligent Transportation Systems (ITSs) have attracted the worldwide interest of researchers, automotive companies and governments. ITSs promise to hugely improve the safety, efficiency and sustainability of the transportation system [1]. A key role in ITSs should be played by the so-called Inter-Vehicular Communications (IVCs), a set of new technologies, standards and protocols that should give full networking capabilities to the vehicles. Roughly speaking, it is possible to identify two main features that IVCs should provide to the vehicles: (i) continuous Internet connectivity on a geographical scale; and (ii) fast, but intermittent, local area connectivity to the vehicles and/or other roadside equipment (e.g., traffic lights, pedestrian crossings, guardrails, _etc._) positioned in a delimited region. The first kind of connectivity could be provided by leveraging existing network infrastructures (e.g., cellular networks), through any devices present in a vehicle, such as dedicated infotainment devices or general-purpose smartphones and tablets. On the opposite side, the local area connectivity is provided through dedicated On-Board Units (OBUs), implementing a set of Dedicated Short Range Communications (DSRCs) technologies, such as the Institute of Electrical and Electronics Engineers (IEEE) 802.11p [2] or the European Telecommunications Standards Institute (ETSI) Intelligent Transport Systems (ITS)-G5 [3] standards, able to provide a fast transport service in a limited geographical area of interest. However, several studies show some important weaknesses affecting DSRC technologies, mostly related to reliability and scalability, making their use impractical in low-density networks or in the case of challenging propagation conditions [4]. Furthermore, as of today, DSRCs are facing a significant resistance in reaching the market, because of their strong dependence on the decisions of public administrations and of the lack of a satisfactory business model for selling the OBUs [4, 5]. In many circumstances, these limitations can be partially overcome by employing cellular technologies, such as Long Term Evolution (LTE) or some kind of third generation (3G) technology, in a complementary manner [6, 7]; even in this case, however, the number of vehicles equipped with an embedded 3G/4G transceivers may be scarce. For these reasons, the general consensus indicates personal devices, such as smartphones and tablets, as key players in developing future ITS solutions. In fact, smartphones are gaining more and more popularity, and nowadays, we may assume that, with extremely high probability, there is (at least) one of these devices inside each vehicle. This new class of personal device is typically equipped with a dual interface, for cellular (typically 3G or LTE networks) and WiFi networks and, therefore, should ideally be able to provide both continuous Internet connectivity and sporadic local connectivity. Obviously, the performance of vehicular communications based on smartphones is not comparable to communications based on dedicated hardware, but in certain conditions, they can offer a satisfying performance level [8]. For all these reasons, it is reasonable to expect that next generation ITSs systems will be based on a heterogeneous mix of hardware (OBUs and smartphones) and communication technologies (DSRCs, WiFi, 3G/4G), as proposed recently in [9, 10]. In this work, starting from the results of [10], we further investigate this kind of heterogeneous architecture, by implementing a smartphone-based service for real-time traffic information dissemination. More specifically, we describe the innovative cross-network approach for effective information dissemination in vehicular environments, proposed in the 2011-2012 Italy-Israel\"Cross-Network Effective Traffic Alerts Dissemination\" (X-NETAD) project [11]. As pointed out in [10], the goal of the X-NETAD project was the design and implementation of low cost, efficient and ubiquitous traffic alerts dissemination to drivers.