**4.1.1 Frontal collision warning**

Frontal collisions represent a major proportion of accidents worldwide. Typical causes of such accidents are the distraction of the driver, sudden braking of a vehicle ahead, the presence of a stationary obstacle in front of the vehicle (e.g. right after a turn) etc. Conventional collision warning systems are based on sensors installed in the vehicle. These sensors could be long range radars for adaptive cruise control, camera sensors for objects detection, cameras covering the blind spot area, laser scanners for both detecting and classifying objects. This way a vehicle can be informed about events and targets which are within range of the detection sensors. Figure 4 shows a frontal collision warning application where the vehicle in front brakes while other vehicles are following.

The reliability and accuracy of a conventional collision warning system is based on the number and the type of sensors used, as well as the type of the environment (i.e. urban, inter-urban, highway) around the vehicle. The occlusion of sensors from obstacles, the limited range of sensors and other physical constraints, reduce system's range and degrade its performance. Apart from these factors, a collision warning system to function properly needs a multitude of sensors to cover the entire area around the vehicle, which makes such a system extremely expensive.

Fig. 4. Frontal collision warning application.

The collision warning system can be much more effective if other neighboring vehicles communicate with the subject vehicle, extending thus the perception of the driver in relation to the limited perception based only on sensors installed in the vehicle. Actually, this is the principle of co-operative collision warning systems. While driving, equipped vehicles anonymously share relevant information, including their position, speed and direction. This way each vehicle monitors the intentions of other drivers and the location and behavior of all vehicles in the neighborhood. When a vehicle detects a critical situation, the system warns the driver with a visual, audible and/or haptic manner. Thus, the driver has enough time to intervene and avoid a collision.

In time critical situations, immediate intervention to avoid a collision is feasible with the use of communication. This would not be possible in case only onboard sensors were used because of the delay in detecting and classifying objects and analyzing the ongoing situation. The co-operative approach also has great influence on the classification of objects. If vehicles are equipped with wireless communication they can directly exchange information about their type (e.g. truck, car, motorbike).
