Road traffic information helps a driver in selecting an appropriate route to the travel destination by avoiding congested road segments. A vehicular ad hoc network (VANET) offers an alternative approach to collect road traffic information without a wired infrastructure and centralized storage. This paper proposes the road traffic collecting (RTC) protocol to gather road traffic information in a VANET on city roads. The key protocol operation is the query dissemination that steers a query message over a set of predefined routes towards the destination location. To collect road traffic information, nodes participating in the query dissemination include their own velocity to the query message so that congestion information on different positions of road segments is gathered. A node nearest to the destination that receives the query message returns the query reply containing road traffic information of the forward route back to the source. Unlike existing broadcasting and geocasting protocols, our proposed protocol uses the message exchange pattern that increases the reliability of query delivery under a collision channel. The simulation on a large network with high vehicle densities and simultaneous query sessions is conducted to evaluate the protocol performance in terms of the percentage of road segment coverage and the query response time under the presence of MAC layer collisions. The results show that RTC can collect 100 % of road traffic information over a set of predefined routes under a few number of parallel query sessions while an existing protocol that mainly uses a broadcast message can achieve only 40-70 %. For a large number of query sessions, RTC achieves 65-80 % of target road segments depending on the vehicle density with the average query time bounded to a few seconds.

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