Travel Time Reliability and Level of Service

Abstract

Travel time reliability (TTR) is familiar to travelers, and its indices are useful measures of the quality of a freeway’s service. Technical groups such as highway agencies are more familiar with a freeway’s level of service (LOS), but the LOS does not capture the variability in travel time. Similar to Pu (2011) this thesis introduces a modified buffer index (BI) incorporating a median rather than average travel time as a new travel time reliability measure. Current research by the SHRP 2 L08 Project Team defines freeway reliability LOS by determining equivalent travel time index for similar travel speed ranges shown on page 8 in TABLE 2. It is anticipated that the new index will be able to provide an additional model for defining LOS using TTR as a service measure on freeway corridors. Data from the BlueTOAD (Bluetooth Travel-time Origination and Destination) devices, which utilize Bluetooth technology, was used to develop a methodology to determine a LOS of the highway facility with the use of the new TTR index and a section of the I-12 highway was used for this study. The Wilcoxon Signed Ranked test was used to assess whether the difference between the median values of the new and existing BIs at various speed ranges are significant. From the Wilcoxon Signed Ranked test the difference between the new and existing BI was found to be significant for speed ranges sr; sr ≥ 60mph, 50≤sr≤59, 45≤sr≤49, 40≤sr≤44, 35≤sr≤39, but not sr < 35mph. However the BI was inconclusive in providing an accurate measure for defining LOS. This is due to the data showing an increasing linear upward trend from LOS A to LOS C but then starts decreasing linearly downwards from LOS D to LOS F. More analysis is needed at all the speed ranges and should be carried out on several segments along the I-12 corridor instead of one segment in order to obtain comprehensive results. Data, including volume data for an entire year is also needed to comprehensively analyse the LOS.