The Impact of Road Alignment Combinations on the Traffic Crash Rates on the Semarang-Solo Toll Road, Indonesia

Road geometry is a crucial determinant of traffic safety, and combined horizontal–vertical alignments can substantially influence crash occurrence. This study investigates the effect of six alignment combinations: Straight–Flat, Straight–Uphill, Straight–Downhill, Curved–Level, Curved–Uphill, and Curved–Downhill, on crash frequency using data from the Semarang-Solo Toll Road, Indonesia. A total of 217 crash records (120 on Route A, 97 on Route B) were analyzed using chi-square tests, linear regression, and generalized linear models such as Poisson and Negative Binomial (NB). The Chi-square analysis revealed a significant association between alignment type and crash frequency (χ² = 28.45, p < 0.05). Straight–Level segments recorded the highest crash share (47%), while Straight–Downhill and Straight–Uphill followed with 18.9% and 16.6%, respectively. Overdispersion (Pearson χ²/df ≈ 7.63) justified the use of the NB model, which outperformed Poisson (AIC = 91.96 versus 104.54). Spatial analysis confirmed differing geometric risk patterns between routes, with Route A dominated by Straight–Flat crashes and Route B by Straight–Downhill. Furthermore, a multivariate analysis revealed that Straight–Flat segments were predominantly associated with rear-end collisions under clear weather (26.7%), while Straight–Downhill and Curved–Downhill sections exhibited elevated single-vehicle crash risks during rainfall. These findings indicate that crash occurrence is jointly influenced by geometric configuration and environmental conditions. These results demonstrate the influence of combined geometric configurations on crash frequency and provide empirical insights for improving design consistency and targeted countermeasures in Indonesian toll road networks.