Statistical modelling of crash frequency on rural freeways and two-lane highways using negative binomial disribution

This study investigated the effects of highway geometric design and other related factors on frequency of rural highway crashes. Highway crash data from Kansas Accident Reporting System database combined with highway geometric data from Control Section Analysis System database were analyzed and modeled using two different model formats. Negative Binomial models were found to be more effective in modeling crash frequencies especially since the dataset was over-dispersed. Different models were developed based on yearly and 5 year average crash data based on 1998 – 2002 time period for rural two-lane and freeway sections. In addition to modeling total crash frequency, Equivalent Property Damage Only crash frequency was also modeled to capture any effects due to severities of crashes. Based on model fitting statistics, it was found that the models based on yearly crash data were better capable of modeling crash frequency compared to models based on average crash data. Model results showed that amount of traffic, speed limit and highway geometric characteristics such as steep sideslopes, grades and sharp curves tend to affect the occurrence of crashes on rural highways. In addition, divided two-lane highways seem to have fewer number of crashes compared to undivided sections and two-lane sections without any access control experience more crashes compared to sections on which access is partially or fully controlled.