This case study of a no-notice evacuation examined the impact of allowing transit signal priority on the evacuation clearance time of transit vehicles and personal vehicles.
The study area was a 14-intersection corridor located in the Southeast corner of Central Washington, DC (NW 7th Street from SW E Street (South) to NW Pennsylvania Ave, West to NW 12 Street). The corridor encompasses a major metro station in the city (L'Enfant Plaza), and is one of the 19 major corridors designated as a primary evacuation route to assist in the evacuation process. The scenario was the detonation of a dirty bomb at L'Enfant Plaza, setting in motion the city's emergency evacuation response.
The methodology used a microscopic traffic simulation of an evacuation environment merged with a transit operations and signal priority component. The evacuation environment consisted of socio-economic data, census data and regional evacuation data, and the transit operations and signal priority component was built from data on street geometry, signal timing data, traffic counts and transit information (schedule, stop location, dwell time, etc.). These models generated an evacuation origin destination (O-D) matrix to create a realistic emergency evacuation traffic model with measures of effectiveness (MOE’s) including travel time, evacuation clearance time, and delay time. The simulation network included 17 of the 34 bus lines within the borders of the study area. The bus lines not included were those that do not require priority (right hand turns only) or do not use more than one intersection within the study corridor.
Allowing transit signal priority during the evacuation resulted in a 26 percent time saving for transit buses, meaning that three prioritized vehicles accomplish the same as four would without priority. The 26 percent time savings enables more transit units to make additional trips, resulting in shorter evacuation times. The results also found that the time saving is achieved without having an impact on evacuation clearance times or evacuee travel times for non-transit vehicles. Moreover, when transit signal priority is restricted to operate only on evacuation routes, evacuee travel and delay time decreases (in contrast to previous studies that found transit priority results in delays to vehicular traffic during high roadway demand).