Relationship between Level of Service and Traffic Safety at Signalized Intersections: Grouped Random Parameter Method

Abstract

Level of service (LOS) is a measure recommended to evaluate the operational performance of roadway facilities. It is generally agreed that LOS is related to traffic safety at signalized intersections; however, this relationship has not been fully investigated. This study sought to examine the relationship between LOS and safety by looking at types of crashes on approaches to signalized intersections. Crash data were acquired from 164 four-legged signalized intersections and categorized by rear-end, left-turn, and total crashes based on vehicle maneuvers. It was observed that intersection approach LOS and crash type frequencies differed for morning peak, midday, and afternoon peak hours. This led to the development of two models: a grouped random parameter negative binomial model to estimate the LOS–safety relationship for total crashes across different time periods, and a bivariate grouped random parameter negative binomial model to estimate this relationship for rear-end and left-turn crashes. Considering that geometric design, signal control, traffic flow, and crash types vary across different approaches within any given intersection, the models were developed at the approach level. The frequency of total and rear-end crashes (most to least) were LOSs D, C, B, and A on signalized intersection approaches. Left-turn crashes were not significantly associated with LOS. The relationship of a specific LOS to rear-end and total crashes varied across different time periods. This study determined that an understanding of the LOS–safety relationship at the intersection approach level requires examining different crash types and different time periods.