Collision probability in Vehicular Ad-hoc Network (VANETs) has been widely studied because it is an essential issue for any performance evolution, such as delay evaluation or throughput. Indeed, EDCA (Enhanced Distributed Channel Access) represents the distributed channel access mechanism in IEEE 802.11p standard. In this paper, we present a new model to estimate collision probability based on the existing model. We use the simulation to study the impact of mobility on EDCA and to compare our proposed approach with those introduced in literature.