Queue predictions at temporary work zones: modification and testing of an existing spreadsheet

Abstract

At the Alabama Department of Transportation (ALDOT), the tool used by traffic engineers to predict whether a queue will form at a freeway work zone is the Excel-based "Lane Rental Model" developed at the Oklahoma Department of Transportation (OkDOT) and whose work zone flow capacity values are based on the 1994 Highway Capacity Manual (HCM, 1994). The scope of this research pertains only to the queue estimation worksheet of that spreadsheet tool, herein referred to as the OkDOT baseline version. This tool, based on input-output logic, is simple to understand and use. Preliminary testing of the OkDOT baseline spreadsheet confirmed a lack of accuracy, and an opportunity to update the capacity estimation method while keeping the rest of the tool intact. Two other versions were created using the work zone lane capacity model of HCM 2000: the HCM 2000 version uses work zone intensity effects of -160 to +160 passenger cars per hour per lane (pcphpl), as prescribed in HCM 2000; the second modified version uses work zone intensity penalties of -500 to 0 pcphpl, a modification based on recent literature, and is, therefore, called the HCM 2000 hybrid version. Although work zone capacity estimation has been widely researched over the past three decades, only a few studies measured actual queue start times, queue lengths (hence maximum queue length); almost all utilize the free flow traffic volume approaching the work zone and predict the capacity of the work zone (rate of traffic exiting the downstream end of the work zone). One in particular (Sarasua et al. 2006) collected extensive data on lane capacity and queue characteristics (if a queue formed) at 35 freeway work zones in South Carolina. We use 32 of these work zone descriptions as the "test data bank" for comparing predictions produced by three versions of the OkDOT spreadsheet tool with the actual maximum queue length (MQL) and queue start time (QST). Minimizing the prediction error in MQL is the main criterion for comparing the accuracy of the three OkDOT model versions, though QST was also considered. Based on the analysis and evaluation, the strong conclusion is that the current tool should be replaced by the HCM 2000 hybrid version we have developed and tested. The HCM hybrid version minimized error in predicting actual MQL at the 32 SC work zones, and minimized the error of not predicting a queue, when one actually formed. Additional testing revealed that a passenger car equivalent PCE = 2.1 for heavy vehicles minimized error in MQL among typical PCE values in the range [2.0, 2.5]. This tool was validated using six work zone cases, three from Alabama and three from North Carolina. In addition to modification of the capacity estimation method in the OkDOT tool, we endeavored to make it more useful for mobility impact assessment by including a graphical depiction of the queue profile. A CD with the HCM 2000 hybrid version of the software was delivered to ALDOT with a final report. A detailed user's guide was prepared and is included as an appendix to this thesis.