Vehicular Flow Past Incidents Involving Lane Blockage on Urban Roads: A Preliminary Exploration

The effects of freeway lane reductions on roadway capacity have been discussed in the literature, but only limited attention has been given to capacity reduction on urban arterial roadways. Videotaping of incident management on arterial roadways as part of another project made available data that could be measured and converted into headways. Fifteen incidents involving crashes and disabled vehicles from which measurements were drawn are examined in this report. All events blocked one lane on a four-lane urban arterial. From the data, headways were measured and estimates of vehicle flow rates computed. Overall, an incident blocking one of two lanes on an urban roadway will reduce the idealized flow from the 1,900 vehicles per hour per lane (3,800 vehicles per hour) as suggested in the Highway Capacity Manual by more than 60 percent for crashes and more than 50 percent for disabled vehicles. Crashes resulting in personal injury had a worse-case rate of 1,230 vehicles per hour, or 32 percent of capacity. The lowest recorded flow rate for disabled vehicles was 1,650 vehicles per hour, or 43 percent of capacity. Distribution of headways around crashes were substantially skewed. On the other hand, headways for disabled vehicles tended to cluster around the mean. Differences in headways not only arose from the type of incident but also appeared to be related to the amount of responder activity (e.g., one police officer versus multiple public safety responders) and by the cross-section of the roadway. In the latter case, roadways with painted medians had shorter headways than those with raised medians or painted center lines. More observations need to be made to validate these initial results.