The connected car vision where vehicles are able to seamlessly communicate to each other and the control centres information about traffic, road and weather conditions will require a large scale deployment of in-vehicle communications technologies. Getting the full benefit of the connected car vision might only require that a small percentage of vehicles be equipped with such communication technologies (as information generated by neighbouring vehicles will be redundant). However, given the numbers of cars on modern roads (30 million vehicles in the UK alone) networking even a small percentage of them would involve large absolute numbers. In order to better understand the communication requirements and dynamics of such a large scale deployment this paper presents the simulation techniques that we have developed under the Traffimatics project. Vehicular dynamics pose challenges to ad hoc communication. Traffic moves unpredictably, in terms of speed and traffic density. Road topologies also vary tremendously and constrain traffic movement and radio propagation. In this work, we have addressed both infrastructure-mediated communications, where wireless access points act as gateways, and infrastructure-less communication where information propagates purely from car to car. Two complementary sets of large-scale simulations have been carried out.