Methodology Update for Estimating Volume to Service Flow Ratio

Volume/service flow ratio (VSF) is calculated by the Highway Performance Monitoring System (HPMS) software as an indicator of peak hour congestion. It is an essential input to the Kentucky Transportation Cabinet’s (KYTC) key planning applications, such as highway adequacy rating. After the Federal Highway Administration (FHWA) reassessed the HPMS, the standalone HPMS software was no longer supported. This conclusion was based on the 2000 Highway Capacity Manual (HCM2000). As a result, the peak capacity and VSF estimates based on the submittal file are no longer available to KYTC. The calculation of VSF ratios requires annual average daily traffic (AADT), K factor, directional factor, and peak capacity as inputs. Previously, peak capacity was calculated by the HPMS software. The calculation method had limitations; it did not take into consideration certain information that may affect the VSF, such as the availability of a truck-climbing lane. Researchers in this study developed a Microsoft Access-based tool for KYTC that estimated peak capacity and VSF. They adapted the approach to ensure compliance with HCM2010. The tool required pre-processing the Highway Information System (HIS) extract to eliminate empty cells by either excluding segments with missing data or by imputing the values of the missing item(s). Often, a short segment was misclassified as a multilane highway or an urban two-lane or three-lane facility. Analyses indicated that VSF can provide a modest amount of information on a segment’s level of traffic congestion. However, it does not reflect variations in travel time (or speed), which may greatly contribute to user experience on the highway. A post-processing step may also be needed to mitigate the impact of short segments in the input data. The future of VSF as the sole measure of congestion performance should be evaluated.