S. Heidi Khazraee
About the research
Several days of traffic data were recorded at a long-term work zone near Pacific MO in both eastbound and westbound directions. A total of 11 breakdown events were identified using average speed profiles. The traffic flows prior to and after the onset of congestion were studied. For this work zone site, breakdown flow rates ranged between 1194 to 1404 vphpl, with an average of 1295 vphpl, while the mean queue discharge rate was equal to 1072 vphpl. The difference between mean breakdown and queue-discharge flow rates is due to the fact that flow usually drops once traffic breaks down and queues start to form. It is suggested that breakdown flow definition should be used in estimation of capacity if congestion is to be avoided, whereas mean queue discharge definition should be used if delays are to be estimated. Most of the breakdown flow rates found in this study exceeded the maximum capacity value used by Missouri DOT for prevention of congestion i.e., 1240 vphpl. The capacity value defined in terms of mean queue discharge as used by HCM 2000 was 1199 pcphpl, which is well below the HCMs average capacity of 1600 pcphpl. This reduction in capacity is attributed mainly to reduced lane width. Additionally, it can be attributed to higher percentage of heavy vehicles, around 25% in the traffic stream.