Traffic flow characteristics and capacity in police-enforced and intelligent work zones

The effects of police enforcement on vehicle speeds in freeway work zones were examined in several studies. Likewise, many past studies investigated the effects of ITS implementation on vehicle speeds in freeway work zones. However, the effects of police enforcement and ITS implementation on work zone traffic flow characteristics and capacity have not yet been extensively studied. Hence, this study investigates the distinct effects of police enforcement and ITS implementation on work zone speed-flow curve and capacity. Three sets of traffic data were collected in a two-lane-open work zone located on I-55 near Chicago. The first data set was collected when there was only the traditional Manual of Uniform Traffic Control Devices (MUTCD) signage in the work zone. The second and the third data sets were collected when there was additionally police enforcement and Speed Photo Enforcement (SPE) in the work zone, respectively. The results showed that both the police enforcement and SPE led to significant changes in the work zone speed-flow curve compared to only MUTCD signage conditions. The general shape of the speed-flow relationship inside the work zone was similar to the speed-flow curve under basic freeway conditions, but the bending point of the upper branch of the curve occurred at lower traffic flow rates and the rate of the decrease in speed was higher. The speed-flow curve for the MUTCD signage-only case returned a work zone capacity of around 1,850 passenger cars per hour per lane (pcphpl). Compared to that speed-flow curve, both the police enforcement and SPE moved the upper (uncongested) branch of the speed-flow curve downward, which caused a work zone capacity reduction of about 50 pcphpl in the case of police enforcement and 100 pcphpl in the case of SPE implementation. The results obtained through this study reveals the distinct effects of police enforcement and ITS on work zone capacity. Accurately estimating the capacity of work zones with different speed reduction treatments provides more efficient operation in a real-time system, more accurate diversion and traveler information for alternate routing, improved reliability of the system, and better understanding of the traffic flow characteristics in work zones.