Modelling the impact of illegal parking on bus priority lane performance: a simulation-based policy evaluation framework

Bus priority lanes are widely implemented to improve public transport efficiency in congested urban environments; however, their effectiveness is often undermined by illegal curb-side parking. Despite this, there is limited quantitative understanding of how such violations influence operational performance under mixed-traffic conditions. This study develops a simulation-based analytical framework to evaluate the impact of illegal parking on bus travel time and effective lane capacity. A microscopic traffic simulation model is used to represent the stochastic occurrence of illegal parking events across varying frequencies, durations, and traffic flow conditions in a controlled environment. The simulation outputs are then used to calibrate a modified Bureau of Public Roads (BPR) function, yielding parameters of α = 3.403 and β = 2.493 (R² = 0.621). The results reveal a threshold-driven, non-linear increase in bus travel time, with longer-duration violations exerting a more significant impact than violation frequency, and travel times increasing by up to 340% under severe disruption conditions. The proposed framework provides a transferable tool for transport planners to quantify performance degradation and supports the development of targeted, performance-based enforcement strategies for maintaining the effectiveness of bus priority systems.