On the Reliability of Safety Message Broadcast in Urban Vehicular Ad hoc Networks

Road safety is one of the most important emerging applications envisioned for Vehicular Ad hoc Networks (VANETs). Generally, such applications involve the broadcast of safety messages, consisting of beacons transmitting vehicles' state (e.g. position and velocity) with a regular period, as well as emergency messages warning about unexpected critical events. From the perspective of safety, the application performance depends foremost on two metrics: for the event-driven warning messages, the probability of message reception; and for periodic messages, the variability of the inter-reception time (IRT), which ultimately determines the freshness of the information received by the driver. In this paper, we develop an analytical model to compute the above metrics in an urban traffic scenario. Focusing on a road segment linked to a signalized junction as a basic building block of urban traffic systems, we apply a novel road traffic density model to investigate the dynamics of the reliability metrics and characterize the region(s) on the road segment according to the achieved safety level. Our numerical study shows that in broadcast mode, the hidden terminal effect is the driving factor determining the reliability of transmissions. Furthermore, the impact of hidden terminals has the greatest effect in road sections where vehicles have high velocity, leading to the poorest performance in regions where reliable reception is needed the most in order to minimize the risk of accidents.