The accident data for Mashhad urban highways in the years 2006
and 2007 were used for modeling accidents on urban highways. This
information was collected by Traffic and Transportation Organization
of Mashhad with the help of GIS and police reports. In this study, the
accidents were divided into two groups: accidents with property
damage only (no injury) and more severe accidents (injury or fatal).
Therefore, accident statistics were collected in two groups: no injury
and more severe. Accident data are usually two-level data, the first
and main level is often the road segment (i.e., the highway is divided
into several parts). In this study, highway segmentation was
performed based on total traffic volume. The second level is the
hour, that is, traffic peak hours are considered the first sub-level, the
day non-peak hours the second, and the night non-peak hours the
third. To scrutinize the roles of the different vehicle types in crash
occurrence or severity, the traffic was separated into passenger cars,
heavy vehicles, and light non-passenger car vehicles. The volume data
were not readily available separately from the Traffic and Transportation
Organization of Mashhad, which was one of the study limitations.
The organization conducts a comprehensive survey once
every few years in November and obtains the passing volume of
different kinds of vehicles through the main urban roads in terms of
daily hours. In November, the traffic conditions in Mashhad are
normal, and the models made in this research do not consider the
seasonal changes of traffic; this could be another limitation of the
research because the statistics of traffic volume in different seasons
were not available, and the volume data are related to different hours
of the day. According to this statistics, one can perceive the traffic
combination and percentage of each vehicle type in traffic and can
trust this combination in the future; however, the total (equivalent)
traffic volume is updated every year.
The Traffic and Transportation Organization of Mashhad gives the
total traffic volume on different urban roads, particularly highways in
peak hours, for future years. According to this total equivalent volume,
the passenger car equivalent factors of vehicle types, the percentage
of each type in traffic combinations, and the ratio of the total volume
in non-peak hours to that in peak hours, the volumes of each vehicle
type in peak hours and 2 hours representing the day and night nonpeak
hours were obtained. The volume of passenger cars and
equivalent volume of heavy vehicles and light non-passenger car
vehicles at peak hours and 2 representative hours of non-peak day
and night hours were also obtained. These calculations were
performed with Excel. The passenger car equivalent factors of vehicle
types are given in Table 1 (Transportation and Traffic Organization of
Mashhad, 2008).