Analysis of geometric design factors influencing driver behaviour and road safety: a case study of Rwanda’’

Abstract

This study investigates the influence of geometric design parameters on driver behavior and road safety along the national road linking Kigali to the Western Province known as the Kigali– Rubavu road (NR2) which is classified as a Class II Road according to the RTDA Road Geometric Design Manual (2014). A total of 40 road segments (20 curved and 20 straight) were assessed through a combination of geometric compliance analysis, statistical modeling, and user perception surveys to understand how design features affect safety outcomes. The study was guided by the following objectives: (1) to assess the compliance of road geometric elements with RTDA and AASHTO standards; (2) to determine and quantify the impact of geometric parameters such as curve radius, gradient, shoulder width, and lane width on accident frequency using statistical models; (3) to investigate how these design features influence driver behavior and comfort, particularly regarding perceived risk and speed management; and (4) to evaluate the role of geometry in the frequency and severity of accidents on Rwandan national roads. Geometric assessment revealed widespread non-compliance, especially among curved segments, where substandard curve radii, excessive Superelevation, steep gradients, and limited visibility were prevalent. Notably, the posted speed limits of 60–80 km/h along NR2 exactly match the design speed range recommended for this terrain classification, providing no additional safety buffer. Field observations confirmed that many drivers exceeded these limits shortly after posted signs particularly in the areas without Speed Camera Stations indicating issues with compliance and enforcement. Quantitative analysis using SPSS showed that a multiple linear regression model explained 93.8% of the variation in accident frequency, with driver comfort, lane width, and shoulder width emerging as significant predictors. Poisson regression analysis confirmed the influence of geometric factors, though with marginal statistical significance. Surveys of 100 drivers and 45 road design engineers further emphasized that sharp curves, steep slopes, and visibility limitations are critical safety concerns. Engineers also reported that accident data and sight distance considerations are often overlooked in practice. iv The findings indicate that several segments along NR2 are poorly designed and fall short of compliance, contributing to elevated crash risk. The study recommends enhancing geometric design enforcement, adjusting posted speed limits to include a safety margin, incorporating perception and crash data into the design process, and addressing geometric deficiencies particularly in curved and mountainous terrain to improve road safety in Rwanda. Furthermore, the study provides a foundation for future research to extend the analysis to other major highways across Rwanda, enabling comparative insights and broader policy implications.