Analysis of Geometric Design Factors Influencing Driver Behaviour and Road Safety: A Case Study of Rwanda

This study investigates the influence of geometric design parameters on driver behaviour and road safety along the Kigali–Rubavu road (NR2), a Class II National Road according to the RTDA Road Geometric Design Manual (2014). A total of 40 road segments (20 curved and 20 straight) were assessed using geometric compliance analysis, statistical modelling, and user perception surveys to determine how design characteristics affect accident frequency and driver behaviour. The geometric compliance analysis revealed widespread non-conformance, particularly on curved segments where substandard curve radii, excessive superelevation, steep gradients, and limited visibility were common. Statistical analysis confirmed that accident frequency was strongly influenced by geometric parameters. For curved segments, curve radius (r = –0.844, p < 0.01) and visibility (r = –0.897, p < 0.01) exhibited strong negative correlations with accident count, indicating that flatter curves and better visibility significantly reduce crash likelihood. The multiple linear regression model (R² = 0.817, p < 0.001) identified curve radius and visibility as the dominant predictors, with increases in each reducing predicted accident frequency by approximately 0.001 and 0.018 per unit, respectively. For straight segments, gradient (r = 0.933, p < 0.01) showed a very strong positive correlation with accident count, while visibility (r = –0.799, p < 0.01) had a strong negative relationship. The regression model (R² = 0.947, p < 0.001) confirmed gradient and visibility as significant predictors, with each 1% increase in gradient associated with a 0.393 rise in accident count, and each additional meter of visibility reducing accident likelihood by 0.004. Poisson and Negative Binomial regression models supported these directional effects but showed limited significance due to small sample size and overdispersion. Survey responses from 100 drivers and 45 design engineers highlighted that sharp curves, steep gradients, and inadequate visibility are key contributors to crashes, while many engineers reported that sight distance and crash history are often underemphasized in design practice. Overall, the findings indicate that geometric deficiencies—particularly insufficient curve radius, steep gradients, and limited visibility—are critical determinants of accident risk along NR2. The study recommends enforcing stricter geometric compliance, incorporating crash and perception data into design standards, reviewing posted speed limits to include a safety buffer, and prioritizing redesign of deficient curved and mountainous sections. These findings provide a foundation for extending geometric–safety analysis to other Rwandan highways for broader policy and design improvement.