Intelligent transport systems provide a multitude of possibilities when it comes to increasing traffic safety on our roads. Many of the proactive traffic safety applications under development today demand timely and reliable treatment of deadline dependent data traffic. Unfortunately it is not possible to provide any timing guarantees when using the current IEEE 802.11p standard for wireless access in vehicular environments. Additionally, a difficult wireless channel environment makes successful data transmissions very challenging. We suggest adding a real-time layer, comprising a deterministic medium access control protocol and transport layer retransmissions, on top of IEEE 802.11p in order to enable guaranteed real-time behaviour and to improve reliability. In a simulation study we show that the packet error rate can be decreased by several orders of magnitude while being able to guarantee timely treatment of both ordinary transmissions and retransmissions by the help of a real-time schedulability analysis.