Estimating the Impacts of Police Enforcement Stops on Freeway Operations and Capacity

Abstract

Over the years, some police agencies have sought to be good stewards of urban traffic flow by reducing their emergency lights while on roadside enforcement stops, particularly during peak hours. Over the past decade, however, move-over laws that require motorists to slow down or vacate the lane adjacent to the stopped emergency vehicle have been implemented in every state. As a consequence, the traffic impacts of police enforcement stops have shifted from a distraction that might slow traffic to a mandate for drivers to change lanes or slow down. From a highway capacity standpoint, the enforcement stop can be modeled as a theoretical lane-blocking event adjusted for motorists' compliance with move-over laws. This research examined 13,416 enforcement stops conducted near roadway traffic detectors on Florida freeways and shows that police enforcement stops reduced the available capacity by between 54% and 58% on two-lane freeways, 35% and 41% on three-lane facilities, 23% and 30% on four-lane facilities, and 14% and 22% when five lanes were present. Regression analysis examined factors to explain the speed of vehicles passing stops and showed that the number of lanes, posted speed limits, and ambient lighting conditions were statistically significant factors. Police agencies should therefore consider implementing policies in which they use their emergency lighting equipment throughout the duration of enforcement stops since they improve compliance with move-over laws and, ostensibly, safety. Breakdowns were not explicitly modeled and are likely not a deterministic measure, so police activity during periods of high traffic flow and congestion should be reserved for compelling traffic safety needs.