Empirical characterization of mass evacuation traffic flow

Among the many gaps in the current state of mass evacuation planning and analysis practice has been a lack of the field-based study of traffic flows under actual evacuation conditions. Without observational studies of such conditions, the simulations and forecasts of roadway performance during emergencies have historically been based on conjecture and professional judgment. However, the recent series of hurricane evacuations along the U.S. Gulf Coast has given investigators the opportunity to collect and evaluate traffic conditions during evacuation scenarios. This paper uses recently collected traffic data from two recent evacuations in Louisiana to assess how well various roadway classifications in different geographic areas were able to carry traffic during emergencies under both normal-flow and contraflow operations. The objectives were to characterize the general conditions of traffic flow under an evacuation condition, to address lingering questions related to maximum sustainable flows, and to examine how the flows compared with those suggested in the Highway Capacity Manual (HCM). The findings of this work suggest that during evacuations most roadways carry flows well below the HCM-predicted maximums and that despite the enormous demand conditions generated by a mass evacuation, the maximum flows on urban roadways typically do not even reach those of typical daily commuter periods.