Finding and Analyzing True Effect of Non-recurrent Congestion on Mobility and Safety

This report summarizes empirical research about the causes and impact of non-recurrent congestion. A method is presented to divide the total congestion delay in a freeway section into six components: the delay caused by incidents, special events, lane closures, and adverse weather; the potential reduction in delay at bottlenecks that ideal ramp metering can achieve; and the remaining delay, due mainly to excess demand. The method can be applied to any site with minimum calibration, but it requires data about traffic volume and speed; the time and location of incidents, special events and lane closures; and adverse weather. The method is illustrated by applying it to a 45-mile section of I-880 in the San Francisco Bay Area. Data limitations preclude applying the method statewide. A simpler method, which depends only on routine data collected in the PeMS system, has been implemented. A PeMS application now provides a ‘congestion pie’ for any district or freeway segment. The pie divides the total congestion delay into three categories: potential reduction, excess demand, and accidents; and an unexplained, ‘miscellaneous’, category.