Flow Characteristics at Freeway Work Zones and Increased Deterrent Zones

Abstract

Traffic-flow characteristics were studied at two work zones on suburban freeways and at a third freeway site where a police patrol car, easily noticed by drivers, was parked on the right shoulder. The purpose of the study was two-fold: ( a) to estimate the speeds, headways, and capacities of sections in which some lanes were temporarily closed to traffic because of relatively short construction operations, and ( b) to assess the impact of increased, visible police deterrents on traffic flow, capacity, and safety. Calculated average flow rates, obtained from 1-min observations, were similar to values suggested by the Highway Capacity Manual at Site 1 but considerably less at Site 2. The lower values at Site 2 are attributed to the transfer of vehicles from the northbound to the southbound lane and to the resulting “constrained” operation that resulted because of a temporary barrier near oncoming traffic. The presence of a police patrol car on the shoulder at Site 3 created a deterrent to drivers, resulting in reduced average headways (and therefore increased density) and reduced headway variability. Because the volumes remained about constant, the resultant space-mean speeds also dropped. The importance of the flow stability—obtained by warning and advisory signing, the gradual closure of lanes, and a visible deterrent such as a police presence—is emphasized. Further research is suggested on the optimal slope and length of lane-closure tapers on freeway work zone approaches, the capacity of other combinations of lane closures, and the safety effects of traffic deterrence caused by police presence.