In the development of Cooperative Adaptive Cruise Control (CACC) systems, spacing policies are primarily developed for optimisation of string stability and traffic stability. However, the safety issue is hardly taken into account. Uncertainty in the communication network and sensor information makes deciding upon a safe minimal headway a non-trivial task. In this paper, we propose a model that is able to approximate the minimal safe time headway, given uncertainty of parameters with varying velocities. By simulating emergency stops, we use the difference in displacement of the cars and a desired maximum probability of a crash to approximate the minimum time headway that yields this probability of a crash. The resulting method is necessary for platooning, a major research development in vehicular networking systems.