Contention-based Forwarding (CBF) is a broadcasting technique used to disseminate emergency messages for traffic safety applications in Intelligent Transportation Systems (ITS). Its design hypotheses have however been based on three major assumptions: uniform vehicular topology, nonfading channels and homogeneous communication capabilities. Realistic vehicular urban topologies do not comply with any of them, making CBF select
relays, which may not exist, may not be reached or may not be optimal due to heterogeneous transmit capabilities. In this paper, we propose to adapt CBF to such challenging environment by first employing two different mechanisms as a function of the topology, and second by considering the dissemination capabilities of the relays, allowing for example road-side units or tall vehicles to preferably act as relays when necessary. Our protocol, called Bi-Zone Broadcast, is evaluated in a realistic urban environment and showed to provide
around 46% improvement in dissemination delay and 40% reduction in overhead compared to plain CBF or flooding. We finally shed light to other aspects of CBF that remain unsolved and should be addressed in future work to further improve the reliability of dissemination protocols for traffic safety protocols.