Vehicular ad-hoc networks, where traffic information is distributed from many sources to many destinations, require data authentication mechanisms to detect any malicious behavior of users, such as modification or replay attacks. In this paper we propose a new data aggregation protocol that uses probabilistic verification to detect such attack attempts a posteriori in an efficient way, with minimal overhead and delay. The algorithm also contains an additional security mechanism based on reactive groups created on demand, which ensure a priori that vehicles generate trustworthy information. According to a comprehensive analysis including both a small-scale real device implementation and NS2 simulations, it is shown that the proposed protocol is robust.