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and intersection collision warning. The objective of these applications is to use IVC to collect surrounding vehicle locations and dynamics and warn the driver when a collision is likely. There are two approaches to collision warning systems (CWS) [41]. In passive approaches, all vehicles must maintain accurate knowledge of all vehicle positions. The risk of a collision is assessed using algorithms that analyze the gathered data (position, speed, acceleration, and direction) [42]. In active approaches, warning packets are sent only when emergency events occur. The emergency event is detected using on-board sensors. It is likely that a practical system will combine both approaches. The unreliable nature of wireless communication and the fast changing group of affected vehicles create challenges in satisfying the strict latency and reliability constraints in CWS. As a result, CWS rely on repeated broadcast as a delivery mechanism for warning messages [39, 43]. Unlike unicast communication where reliability is enhanced by receiver s feedback (e.g., via RTS/CTS handshake), it is dif cult to identify all receivers of a broadcast message and obtain their feedback in a highly mobile network. To enhance probability of successful reception, without receiver feedback, several copies of each message are transmitted without acknowledgment in combination with the CSMA mechanism of IEEE 802.11 [40, 44, 45]. A priority access mechanism can also be used to improve the probability of successful reception in a saturated medium. Moreover, the priority mechanism allows a quicker access to the medium compared to nonprioritized mechanisms [40, 46, 47]. Due to the broadcast nature of Emergency Warning Message (EWM) transmissions, it is not possible to control the rate of transmission based on channel feedback. Instead, application-speci c properties are used for EWM congestion control [39]. In the Vehicular Collision Warning Communication (VCWC) [39], a control policy is proposed to reduce the transmission rate of the warning messages as more vehicles react to the initial broadcast by starting their own broadcast. Another proposed method in reference 39 eliminates redundant emergency warning messages by exploiting the natural chain effect of emergency events. 14.7.2 Automated Highways and Cooperative Driving This type of application is concerned with the automation of some driving functions in order to increase driving safety and improve the capacity of highways. Among the applications considered are assisted/automated takeover and lane merge, platooning, automatic cruise control, and emergency vehicles announcement. These applications are responsible for the exchange of the drivers intentions, signals, and data on varying relative positions [48]. With the help of an infrastructure that provides V2R communication, other applications can be included such as hidden driveway warning, electronic road signs, intersection collision warning, railroad crossing warning, work zone warning, highway merge assistance, and automated driving. Among the early projects in IVC are those that investigated automated car platooning. The principal motivation for this application is increasing highway capacity. Essentially, the capacity can be increased substantially by reducing the spacing
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between vehicles traveling at high speed. Since drivers reaction time is too slow to achieve this goal safely, vehicles must operate under automatic control [49]. Notable examples of automated platooning are the California PATH project [50], the European Chauffeur project [51], and the Japanese DEMO 2000 [52]. In these projects, the vehicles were able to perform several tasks, such as lane merge, lane split, and avoid an obstacle. The IVC played an essential role in the vehicle control because the necessary data for the platooning had to be exchanged among vehicles over the communication links. Safe platooning imposes the most severe conditions on the IVC. Thus it requires a high-speed wireless communication network capable of delivering reliable and fast messages [48]. One of the challenges facing safety and cooperative driving applications is the coexistence of equipped and nonequipped vehicles, which may affect the function of these systems. Since it is not realistic to expect that all vehicles are equipped at the early stages of this technology, a candidate solution may be the fusion with other systems. For instance, collision avoidance at intersections and similar applications could be realized with the assistance of V2R communications [53]. Another solution may rely on an Automated Highway System Architecture (AHS) [50]. 14.7.3 Local Traf c Information Systems Established traf c information systems have been traditionally based on centralized architectures. Roadside sensors deliver traf c data to a central unit where the information is processed. The traf c information is then disseminated to drivers via radio broadcast or on demand via cellular phone [54]. By utilizing on-board sensors, the GPS, and digital maps, a powerful traf c information system can be realized using a VANET that can be rapidly deployed without the need for much of the expensive infrastructure that is required in existing traf c and travel information systems [55]. Information systems that make use of IVC face two major challenges: required market penetration and scalability. With the initial low market penetration of equipped vehicles, the effectiveness of these system is limited. However, it is argued that an effective system can be created even if only 1 3% of all vehicles are equipped with an IVC system [54]. Scalability becomes an issue once a higher market penetration is reached due to data overload conditions. Traf c information is most likely to in uence drivers decisions when it relates to the immediate area or an area they are likely to enter. Therefore, the general approach to the scalability challenge is to increase the level of abstraction of the collected data as the information is propagated further from the source. Fore instance, periodical data reports are forwarded, unmodi ed, over a xed number of hops, and then only particular results from the on-board traf c analysis is forwarded further [56]. Similarly, traf c information will only be transmitted to a vehicle if that information is relevant in terms of the vehicle s likely route [55]. In the system proposed in reference 54, scalability is achieved by dividing each road into segments of limited size (application dependent). Each vehicle monitors the locally observed traf c situation using its own sensors and by recurrently receiving data packets with detailed information from other vehicles. A traf c situation
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