Factors Affecting Reliability of Urban Bus Services

Attitudinal surveys have indicated that travel time reliability is an important attribute of urban transportation services affecting choice of mode of travel. Yet there is no generally accepted measure of transport reliability, nor is much known about factors affecting this parameter. This study was conducted to test a particular measure of reliability, the inverse of the standard deviation of point-to-point travel times, using data from bus services in the Chicago area. The selected measure was found to be a useful and easily collected indicator of service reliability. Reliability measured in this form was found to be significantly degraded by increasing route length, increasing intensity of intersection control (particularly traffic signal density), increasing traffic volumes, and, with less certainty, increasing bus passenger loadings. Several strategies for increasing urban bus transit reliability are suggested.