Vehicular ad hoc networks (VANETs) are emerging as a new network environment for intelligent transportation systems (ITS). In many applications envisaged for VANETs, traffic information needs to be disseminated to relevant vehicles during a duration of time. Here we present a model of warning message dissemination in VANETs. The idea in our model is that vehicles on a stretch of roads carry warning messages to vehicles in the opposite direction, so as to warn them regarding a dangerous situation ahead. The objective is that as much as possible of the vehicles in the opposite direction receive the warning before they reach the warning line, whereas as few messages are broadcasted as possible. In this model, we mainly use vehicles traveling in opposite direction to reduce broadcast overhead and help message delivery upstream. Furthermore, we dynamically adjust the effect distance of individual relay vehicles through predicting the interval of vehicles in one lane, thus the relay vehicles can stop broadcasting in time in order to reducing unnecessary broadcasts while keeping the warning message in the affected area. Simulations show us the quality of the proposed model by measuring how many affected vehicles are informed in time and the broadcast overhead under various conditions.
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