Quantifying the Effects of Modal Conflicts on Transit Reliability

Residents of San Francisco, California, have repeatedly cited transit reliability as one of their top concerns. As San Francisco has begun investing in projects to reduce travel time variability, the trade-offs between alternatives have been difficult to quantify with existing tools. This study investigated and quantified the effects of modal conflicts on bus travel along a 3.6-mi study corridor and each conflict's contribution to unreliability. Instances of modal conflict were modeled as Bernoulli trials or other simple probabilistic events and included vehicles entering or exiting parking spaces, double-parked vehicles, and vehicles queuing for pedestrians while executing turning movements. Cumulative distributions of modal conflict resulted in estimates of average delay and variance of delay from bus to bus. Sensitivity analyses illustrated and quantified the nonlinearity and variance of delay with respect to the number and severity of events. The convolution of distributions of delay for each conflict type could also be calculated, provided the causes of delay were independent. Modal conflict delay for a side-running bus rapid transit line in the study corridor was 1.9 min longer for buses with 95th percentile delays than for those with the mean delay of 4.2 min. Overall, the semi—standard deviation of delay compared with a no-modal-conflict reference point was 4.3 min. Furthermore, eliminating the analyzed modal conflict types along the study corridor–for instance, by establishing center-running bus-only lanes–could reduce system operating costs by up to $2.5 million per year and could result in user time savings equivalent to as much as $10.5 million per year.