used for identifying hazardous sites, predicting motor-vehicle collisions, and developing accident modification factors (Lord & Park, 2008).
In this paper, the role of vehicle type volume in traffic flow in the occurrence of accidents on urban highways is investigated through modeling no injury and more severe accidents by Poisson and NB regression models. The vehicle types are divided into three groups, including: passenger cars; heavy vehicles including truck trailers, trucks, buses, and minibuses; and light non-passenger car vehicles including taxis, pickups, and motorcycles. The purpose of the study was to scrutinize the role of each group of vehicles in the likelihood of accidents. The role of heavy vehicles in accident occurrence is often discussed, but it must be determined whether heavy vehicles result in more accidents or whether passenger cars or light non-passenger car vehicles, like taxis or motorcycles, play a key role in accident occurrence or frequency.