This research report analyses information propagation speed in bidirectional vehicular delay tolerant networks such as roads or highways. The analysis provided in this report shows that phase transition occurs concerning information propagation speed, with respect to vehicle density in each direction of the highway. Under a certain threshold, information propagates on average at vehicle speed, while above this threshold, information propagates faster. This report gives the exact expression of this threshold and of the average propagation speed in this context, around the threshold. Simulations results, obtained on several platforms, are presented and confirm the analytical results.