Vehicle safety communications research is rapidly advancing in Europe and the U.S. from proof of concept to large scale field testing. Congestion control is a key challenge facing the current efforts to develop a deployable system able to function properly once the penetration rate reaches a high level. This paper focuses on how channel load forms in the context of vehicle safety communications. How a transmitting vehicle contributes to channel load is described mathematically. It is then shown that load contributions from all transmitting vehicles can be aggregated to give a complete picture of channel load at all positions on the road. This paper provides a very efficient method to quantitatively analyze channel load given an input of RF model, vehicle distribution and each vehicle's transmission behavior.