Traffic Capacity, Speed, and Queue-Discharge Rate of Indiana’s Four-Lane Freeway Work Zones

In addition to traffic capacity, vehicle speeds and queue-discharge rates at work zones are essential in assessment of work zone traffic delays and user costs. The traffic flow characteristics of freeway work zones were analyzed based on the traffic data collected from Indiana four-lane freeways. It was found that traffic congestion at work zones was characterized by sustained low vehicle speeds and fluctuating traffic flow rates. Therefore, work zone capacity was defined as the traffic flow rate just before a sharp speed drop, followed by a sustained period of low vehicle speed and fluctuating traffic flow rate. Study results indicate that the mean queue-discharge rates in Indiana freeway work zones were lower than the work zone capacities, even though, at times, individual values of queue-discharge rates could be higher than capacities. Therefore, it is not justified to use work zone capacity values, instead of queue-discharge rates, in estimating traffic delays and user costs under congested conditions. Vehicle speeds at work zones under uncongested conditions remained stable and close to the work zone speed limit of 88.5 km/h (55 mph), while they dropped 31.6 to 56.1 percent from the normal work zone speeds during congestion. The values of work zone capacity, queue- discharge rate, and vehicle speed provide valuable input for predicting traffic congestion, estimating traffic delays, and analyzing user costs at work zones.