Car-to-X Communications are envisioned to improve road safety, traffic efficiency, and information services through short-range and real-time systems. The enabling applications have varied requirements such as low latency, specific forwarding patterns, and reliable data exchange between vehicles and infrastructure. Hence, a cross-layer architecture for vehicular applications should be defined according to the goals of the implementation, so as to consider proper routing and dissemination mechanisms, communication protocols, and the application’s design and operation. In this context, we propose the characterization of the upper layers of a safety-oriented Car-to-Car application. We propose a cross-layer application/network layer design for a Post Collision Notification (PCN) application, and by means of a coupled simulation model that combines the communication network with vehicular traffic flow, we assess the performance of the application design. Three main results can be derived from our work. First, the selected geocast protocol (DRG) shows to be an accurate protocol for safety applications and it may be a scalable routing mechanism for other applications. Second, that DRG is effective even for larger urban areas. And third, there is a reduction of acceleration and speed of the closest vehicles to the incident zone, which is a positive impact of the PCN application on the traffic flow. Consequently, the proposed cross-layer architecture and the implementation of geocast routing has shown a suitable support and good performance for safety applications, and could be extended to other application on the Car-to-X domain.