Improvement of traffic safety by cooperative vehicular applications is one of the most promising benefits of VANET. However, in order to develop properly such applications, the influence of the different driving parameters on the event of a vehicle collision must be assessed at an early design stage. In this paper we derive a stochastic model for the number of accidents in a platoon of vehicles equipped with a warning collision notification system, which is able to inform all the vehicles about an emergency event. In fact, the assumption of communications being used is key to simplify the derivation of a stochastic model. The model enables the computation of the average number of collisions that occur in the platoon, the probabilities of the different ways in which the collisions may take place as well as other statistics of interest. Although an exponential distribution has been used for the traffic density, it is also valid for different probability distributions for the traffic densities as well as for other significant parameters of the model. Moreover, the actual communication system employed is independent of the model since it is abstracted by a message delay variable, which allows it to be used to evaluate different communication technologies. We validate the proposed model with Monte-Carlo simulations. With this model one can quickly evaluate numerically the influence of the different model parameters (vehicle density, velocities, decelerations and delays) on the collision process and draw conclusions that shed relevant guidelines for the design of vehicular communication systems as well as Chain Collision Avoidance (CCA) applications. Illustrative examples of application are provided, though a systematic characterization and evaluation of different scenarios is left as future work.

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Abstract-Improving safety on the road is one of the most challenging goals of recent investigations in VANETs (Vehicular Ad-hoc Networks). CCA (Chain Collision Avoidance) applications are a new emerging means of reducing the number of accidents on the road by providing cars with collaborative communication capabilities, thus allowing them to react against the real risk of accident which may occur in different traffic circumstances. In this paper a CCA application is presented, evaluating the performance of a one-hop notification delivery scheme for vehicles in danger of a chain collision. Security improvements when using such a system are revealed, showing that the transition from a scenario with no CCA support on vehicles to other implementing full CCA capabilities on cars will at first yield some difficulties which must be thoroughly evaluated.

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