How does an increased freeway speed limit influence the frequency of crashes on adjacent roads?

Urban arterial roads carry the most traffic on urban road networks and experience the highest percentage of crashes in urban areas. Safety on urban arterials that are adjacent to a higher speed freeway may be impacted by speed spillover or adaptation. The objective of this study was to determine the effects of raising freeway speed limits on the frequency of crashes on urban arterial roads adjacent to freeways (spillover effects). Crash data within Michigan were collected on 1393 urban arterial road segments before and after freeway speed limits were altered. Before-and-after data was collected simultaneously on 1470 comparison segments of urban arterial where speed limits did not change to control for the regression-to-the-mean bias. The mixed effects negative binomial (MENB) regression model was developed to analyze crash frequency on urban arterials. The results indicate that raising speed limits of freeways by as little as five miles per hour had a likelihood of increasing crash frequency on adjacent arterial roads by as much as 13.9 percent. To investigate if the safety impact of speed spillover changes with the distance from the freeway, influence areas (0–1 mile, 1–2 mile, and 2–3 mile) were used. The findings of this study provide insights into the effects of speed spillover on crash occurrences, and it demonstrates that increasing freeway speed limit has a negative influence on driver compliance with the speed limit on adjacent arterial roads. Correspondingly, the influence of freeway speed on drivers’ speeding behavior on adjacent urban arterials fades away with the distance from the freeway.