Flow Characteristics at Freeway Work Zones and Increased Deterrent Zones

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.