Developing distraction-related safety performance functions at interchange ramp terminals in Kentucky

Introduction: The complexity of ramp terminals, coupled with distracted driving (which impairs drivers’ abilities and slows reaction time), presents significant challenges to drivers. The safety at interchange ramp terminals, particularly concerning distracted driving crashes, has not been thoroughly investigated. This study develops safety performance functions (SPFs) for fatal-and-injury (FI) distraction-related crashes at ramp terminals in Kentucky between 2018 and 2021. Method: To account for zero-inflated count data, zero-inflated models, including the zero-inflated negative binomial (ZINB), zero-inflated Conway-Maxwell-Poisson (ZICMP), and zero-inflated heterogeneous Conway-Maxwell-Poisson (ZIHTCMP), were used. Results: The goodness-of-fit measures indicated that the ZIHTCMP model outperformed the other models. From the developed SPFs, the distance to adjacent ramp terminal (> 800 ft) and presence of traffic signal control were associated with increased FI distraction-related crashes. On the other hand, the presence of a channelizing island at the exit ramp, presence of rumble strips at the exit ramp, presence of an exclusive left-turn on the crossroad (or major road), and higher number of lanes at the exit ramp were associated with reduced FI distraction-related crashes. An in-depth investigation of distraction-related crashes at the high-crash ramp terminal locations revealed that “in-vehicle distraction” and “cognitive distraction” were the most common incidents. Practical Applications: Based on the study findings, several targeted countermeasures were proposed to improve safety related to distracted driving at ramp terminals, such as designing ramp terminals within 800 ft of each other, installation of retroreflective backplates on signal heads, and installation of optical speed bars on the exclusive right-turning lanes on crossroads.