A vehicular ad hoc network (VANET) is an emerging technology which has a great potential of new applications in safety, traffic optimization, and entertainment. The VeMAC [1], [2] is a medium access control protocol recently proposed for VANETs, which can support efficient broadcast service necessary for high priority safety applications. The VeMAC protocol reserves disjoint sets of time slots to vehicles moving in opposite directions and to road side units (RSUs). The protocol has been evaluated for vehicle-to-vehicle (V2V) communications and under balanced vehicle traffic conditions, in which the densities of vehicles moving in opposite directions on a two-way road are approximately equal [2]. In this paper we investigate the effects of the existence of RSUs and the unbalanced vehicle traffic conditions on the VeMAC performance, via simulations in highway and city scenarios in terms of network throughput and transmission collision rate.