Abstract
This study develops crash rate prediction models based on the premise that crash frequencies observed from adjacent paired non-weaving and weaving freeway segments are spatially correlated and therefore requires a simultaneous equation modeling approach. Simultaneous equation models for paired freeway non-weaving segments and weaving segments along with combined three freeway segments upstream and downstream were developed to investigate the relationship of crash rate with freeway characteristics. The endogenous variables have significant coefficients which indicate that unobserved variables exist on these contiguous segments, resulting in different crash rates. AADT is a variable that can show the interaction between the traffic and crashes on these contiguous segments. The results corroborate such an interaction. By comparing the simultaneous equation model and the multiple linear regression model, it is shown that more model parameters in the simultaneous models are significant than those from linear regression model. This demonstrates the existence of the correlation between the interchange and between-interchange segments. It is crucial that some variables like segment length can be identified significant in the simultaneous model, which provides a way to quantify the safety impact of freeway development.
Highlights
Geometric conditions on freeways influence the occurrence of crashes
The endogenous variables have significant coefficients which indicate that unobserved variables exist on these contiguous segments, resulting in different crash rates
The freeway segment at an interchange and those between interchanges are different, one with traffic exiting off the ramp, and the other with traffic entering from the ramp
Summary
Geometric conditions on freeways influence the occurrence of crashes. For instance, ramp spacing could be too short to provide sufficient space for drivers to negotiate with other competing drivers for maneuvers such as lane changes. The congestion caused by crashes would remain local This migration is dependent upon whether or not there are traffic controls at the on-ramp entering the segment downstream. Due to the spatial adjacency nature of crash frequency observed, it is imperative that new models are developed that can consider the correlation between crashes in adjacent freeway segments. Simultaneous equations models were adopted to integrate the correlation of crashes on contiguous freeway segments. Three simultaneous equations were developed where the segments downstream and upstream of one freeway segment were considered. It should be noted that some studies have already developed models to address the spatial correlation of crashes on adjacent freeway segments.
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