Safety effects of signalizing uncontrolled intersections on suburban arterials: An empirical Bayes before-after evaluation by crash type.

Suburban arterial intersections often face safety challenges related to high speeds, mixed road users, and incomplete traffic control. Signalization is a commonly used countermeasure, but existing studies have primarily examined the signalization of stop-controlled intersections, with limited before-after evidence for previously uncontrolled intersections. This study used crash data from 2020 to 2025 to evaluate the safety effects of signalization at 17 previously uncontrolled suburban arterial intersections in Guangdong Province, China, with 100 untreated intersections serving as the reference group. An empirical Bayes (EB) before-after approach was used as the primary evaluation method, and a safety performance function was developed using the reference intersections to estimate the expected crash frequency at the treated sites in the absence of signalization. A propensity score matching (PSM)-based cross-sectional analysis was also conducted for comparison. Sensitivity analyses based on covariate calibration and E-values were conducted to assess potential confounding bias in the total-crash estimate. Crash modification factors (CMFs) were also estimated by crash type and examined in relation to changes in average speed. The results showed that total crashes increased significantly after signalization, with an EB-CMF of 1.884. Rear-end, same-direction sideswipe, and non-motorized-user crashes also increased significantly, whereas fatal and angle crashes showed no statistically significant changes. The PSM-based cross-sectional analysis produced a directionally consistent crash frequency ratio of 1.913, although its confidence interval was wider. The sensitivity analyses showed that including population density in the SPF changed the total-crash EB-CMF only slightly, from 1.884 to 1.881. The E-value of 3.174 indicated that relatively strong unmeasured confounding would be required to shift the point estimate to the null. Average speed decreased significantly after signalization, but the exploratory analysis did not identify stable associations between the evaluated average-speed measures and the total-crash or crash-type-specific CMFs. Overall, signalization produced varying effects across crash types, highlighting the importance of integrating signal control with supporting measures for left-turn control, pedestrian and non-motorized-user facilities, and approach channelization.