Transport Infrastructure and Urban Mobility: A Data-Driven Evaluation of Bus Layby Impact on Expressway Efficiency

Abstract

Expressways are designed to facilitate uninterrupted traffic flow; however, localized roadside features such as bus laybys disrupt vehicle dynamics and reduce operational capacity. While classical speed–density models provide theoretical foundations for freeway analysis, localized disturbances are frequently omitted from conventional modeling. This study evaluates the speed–density relationship of passenger cars on a basic expressway segment adjacent to a bus layby along the Federal Highway (KM 17) in Selangor, Malaysia. Traffic data were collected across peak and off-peak periods using elevated video recordings and analyzed via Tracker motion-analysis software across 36 five-minute intervals. Comparative evaluation against classical single-regime models demonstrated that the linear Greenshields model ($V_s = 116.97 - 1.381k$) provided the most robust fit, explaining 75.05% of the variation in passenger car speed ($R^2 = 75.05\%$, $F = 102.27$, $p < 0.001$). The empirical findings indicate that each 10-unit increase in density ($pcu/km$) induces an approximate 14 $km/h$ reduction in speed, largely driven by lane-changing turbulence and deceleration near the layby weave zone. These results underscore the need for infrastructure authorities to incorporate localized disturbance factors into expressway capacity planning and offer actionable insights for optimizing bus layby geometry and setback distances to preserve highway efficiency.