Examining Freeway Bottleneck Features During a Mass Evacuation

Traffic features were investigated for a bottleneck that was observed on a 30 mi northbound section of Florida's Turnpike (SR-91) during the mass evacuation in advance of Hurricane Irma that occurred in September 2017. Radar detector data (at 1 min intervals) from the Regional Integrated Transportation Information System were utilized to determine the periods when a bottleneck was active adjacent to a service plaza along the roadway. Three distinct time periods were identified during which a bottleneck was active at the service plaza off-ramp, for a total of 27.5 h during the evacuation period. To identify and confirm each bottleneck activation and duration, and to measure the traffic flow features that characterized the bottleneck, curves of cumulative vehicle count and occupancy were utilized. Analysis of these curves revealed time periods during which excess vehicle accumulation and delay occurred between successive detector stations along the Turnpike. Results demonstrate distinct queued and free flowing traffic states between adjacent detectors in the vicinity of an off-ramp into a service plaza. The apparent bottleneck discharge features presented substantially lower flows than what would be expected for a limited access facility with high operational speeds. Findings from this paper present important considerations for evacuation planning and modeling as roadway traffic features may only present themselves during evacuations and if not accounted for may drastically reduce the precision of models and simulations.