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 field-based study of traffic flow under actual evacuation conditions. Without observational studies of such conditions, simulations and forecasts of roadway performance during emergencies has historically been based conjecture and professional judgment. However, the recent series of hurricane evacuations along the Gulf Coast have given an 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 are able to carry traffic during emergencies under both normal- and contra-flow operation. The objectives were to characterize the general conditions of traffic flow under an evacuation condition and to address lingering questions related to maximum sustainable flows and examine how they compared to those suggested by the Highway Capacity Manual (HCM). The findings of this work suggest that during evacuations, most roadways carry flows well below HCM predicted maximums and that despite the enormous demand conditions generated by a mass evacuation, maximum flows on urban roadways typically do not even reach those of typical daily commuter periods