## 7 Conclusions and Open Issues  
This paper presents a survey of application of WSNs to ITS. As it has been shown, WSN is a technology which may have a relevant role to ITS, enabling cost-effective and accurate solutions with a wide variety of applications in driving safety and traffic control as well as in parking management. Its contribution is not merely about sensing the environment but about making advanced collaborative ITS applications possible. In this respect, in addition to processing data in centralized TMCs, WSNs can be used to process information _in situ_, reducing data distribution costs and offering a fast response to critical events. The plethora of innovative possibilities that WSNs confer can be further extended if they are complemented with the joint use of other technologies such as VANET or WAN networks, allowing different data dissemination schemes. Therefore, WSNs may take part in heterogeneous ITS in which every adopted technology is used for the purpose it best serves. In spite of this heterogeneity, the design of WSNs for ITS is driven by the same basic premises that any efficient WSN application must satisfy. This is achieved by finding a balance between, on the one hand, low power operation and processing complexity, and, on the other hand, QoS assurance, which is typically accomplished by applying effective duty cycling schemes and task assignment among the nodes. However, these applications differ from other WSNs applications in the additional restrictions that ITS systems impose and in the opportunities they offer. This is mainly due to the mobility of vehicles, subject to relatively high speeds and motion bound by roads. This affects a great number of issues including the detection and estimation of significant features from vehicles, the placement of nodes and the design of routing algorithms, the latter implying a simplification in the development of collaborative applications. In addition, since vehicles pass by sensor nodes in a sequential way, it is possible to develop predictive and adaptive applications as well which correspondingly may learn about the arrival of a vehicle in advance or adapt their action prior to the arrival, thus allowing both power savings and accurate operation. There are however, some concerns and opportunities in the ITS scenario which either have not been addressed yet or have been only partially tackled. One such example is security, which is only rarely included in WSN-based ITS works, and for which there are really few specific research works [17]. Currently, the adoption of existing ITS tailored security mechanisms poses an important increase in energy consumption which makes them unfeasible, thus requiring some improvement. In addition, there is a focus on securing the information originated by roadside WSN nodes, not considering the case where these nodes obtain information from vehicles. In this respect there is a wide range of potentially vulnerable situations not well covered which can be inferred from security works in VANETs [94]. Synchronization is another issue to be considered. In spite of the existence of a great number of works which consider synchronization, there are few specific solutions for the WSN-based ITS case [85]. Nevertheless, vehicular scenarios enable interesting possibilities involving moving vehicles. A very common problem found in synchronization is the cumulative error introduced in each hop when nodes try to synchronize along a multi-hop path. However, in vehicular scenarios, one can take advantage of the moving vehicles to synchronize a whole string of roadside sensor nodes, one by one, using simpler mechanisms and obtaining more accurate results. Multi-purpose systems, in turn, remain as preliminary proposals in several works but without being extensively addressed yet. These systems use a same deployment in order to perform different goals, e.g., traffic safety and traffic control, thus making them more flexible and valuable.