Estimation of traffic impacts at work zones

Assessing the safety and mobility impacts of work zones across the project development phases of road construction and maintenance projects is an emphasis area of the Federal Highway Administration's Final Rule on Work Zone Safety and Mobility1 (Final Rule). Specifically, the design phase of developing traffic control plans requires performing a traffic analysis to estimate queue lengths, travel times, and delays to determine lane closure times. State departments of transportation (DOTs) must comply with the requirements of the Final Rule by October 2007. To this end, this study was conducted to provide the Virginia Department of Transportation (VDOT) with the state-of-the-practice tools that are available and used by other state agencies for estimating the traffic impacts at work zones. The researcher found that all models based on the Highway Capacity Manual (HCM) assume capacity as an exogenous variable that is given as input to the model; delay and queue length are dependent on capacity. A good estimate of the capacity of a work zone bottleneck is essential to obtain an accurate estimate of traffic impacts. The capacity charts in HCM 1994 were determined for work zones in Texas based on studies conducted before 1982. Based on the recommendations in HCM 2000, it is clear that the 1994 capacity charts significantly under-predict the capacity values at short-term freeway work zones. However, it is possible to obtain realistic capacity estimates from HCM 2000 by using base capacity values specific to the state and applying the necessary adjustment factors for intensity of work activity, effect of heavy vehicles, and presence of ramps in close proximity to the work zone. Data intensiveness, level of effort, and accuracy of the estimates are the key elements state DOTs use to choose the tools for traffic impact analysis. It can be safely assumed that most of the HCM-based tools are easy to use, are not data intensive, and generate quick results, with the exception of QuickZone, which could be data intensive and might require greater user effort. Many state DOTs use the size of the project as an element. Comprehensive tools such as QuickZone and microscopic simulation that are highly detailed and incorporate traveler response to the prevailing traffic conditions might be suitable for use for large projects. There is evidence that simple spreadsheet models and the QUEWZ model produce more accurate estimates of traffic impacts than do QuickZone and microscopic simulation. The inability of many available traffic simulation models to model the oversaturated conditions at work zone bottlenecks is one reason for the erroneous estimates. The conclusions in this study should help VDOT choose the appropriate tool(s) for estimating the traffic impacts in and around work zones. This is a very high priority for VDOT's Traffic Engineering Division as it works on the development of an agency-wide plan to comply with the Final Rule for roll out by the end of 2006.