3. Methodology
In this research, the accidents on urban highways in Mashhad were modeled by Poisson and NB regression models, two models for no injury accidents and two for more severe accidents (injury or fatal). The specific contribution of this research is the separation of the total traffic volume into the passenger car volume, the heavy vehicle volume, including truck trailers, trucks, buses, and minibuses, and the light non-passenger car vehicle volume, including taxis, pickups, and motorcycles. The role of volume in the occurrence of accidents with property damage and injury or fatalities was investigated thoroughly to determine what kind of traffic has a significant role in accident occurrence. Thus, the independent variables of both no injury and more severe accident models are as follows:
1.
passenger car volume (Veh/hr)
2.
heavy vehicle volume (passenger car equivalent)
3.
light non-passenger car vehicle volume (passenger car equivalent)
Accident data are usually two-level data; the first and main level is often road segments (i.e., the highway is divided into several parts or segments). The base of segmentation can differ, and this segmentation was based on total traffic volume in this study (i.e., the total traffic volume [passenger car equivalent] is constant for each part of the highway). The second level is the hours of the day, that is, the traffic peak hours are considered as the first sub-level, the day non-peak hours the second, and the night non-peak hours the third. The accident data for Mashhad urban highways were collected by the Transportation and Traffic Organization of Mashhad. After developing the models, the Poisson and NB models for no injury and more severe accidents were compared, and the better model was then validated to prove the efficiency of that model in modeling urban highway accidents.