Deriving Work Zone Capacities from Field Data – Case Studies of I-70 in Missouri

This paper compares work zone capacity values determined from field data using three methods: (1) maximum sustained flow method, (2) re-scaled cumulative flow curves method, and (3) 85th percentile method. To the authors knowledge, this is the first application of the re-scaled cumulative curves method to work zones. Field data was collected at four short-term 2 to 1 work zones (one lane open out of two lanes) of Interstate 70 in Missouri. The study found the maximum sustained flows decreased as the aggregation interval increased from 5 to 15 minutes. The queue discharge flow (QDF) values were the most conservative estimates of capacity. The 85th percentile flows were lower than the 15-min sustained flow values in all but one location where the 85th percentile flow was slightly higher by 4 passenger cars per hour per lane (vphpl). The pre-queue flow (PQF) values, indicative of near-constant flow prior to breakdown, did not occur in any of the four work zones. This finding has implications on the design of traffic control methods to alleviate work zone congestion. The existence of PQF and its corresponding probability of breakdown have been exploited in the design of ramp metering on freeways. The success of such methods in short-term work zones is questionable due to the absence of PQF. The average capacities obtained by averaging values from all four sites were 1301, 1149, 1267 vphpl, for 15-min sustained flow, QDF, and 85th percentile flow, respectively, which were close to the HCM-based capacity value of 1240 vphpl currently used by Missouri Department of Transportation (DOT).