1331098 - Inclusion of Reliability in the Volume Delay Function

Volume-delay functions (VDF) have a key role in processing traffic assignment. They need to accurately present a relationship between travel speed (or travel time) and traffic flow. In traffic assignment step, drivers choose the best travel route based on traffic conditions and travel costs. As an indicator of this choice, travel time is used. Bureau of Public Road’s (BPR), which was developed in 1964 is the most widely used delay function for both uninterrupted and interrupted roadways to compute congested travel times in assignment .However, there are some drawbacks related to the BPR curve. This equation works on V/C less than 1.2 and loses the precision with V/C greater than 1.2 .Considering the current drawbacks of current BPR, it is required to develop a better function that doesn't go infinity after v/c of 1.2 and includes reliability to have estimated travel time closer to the observed one.
This research proposes the volume delay function (VDF) which includes reliability (VD(R)F). The VD(R)F tests a reliability term that is a function of the current VDF while the VDF has traditionally been considered the Travel Time Index (TTI). The Buffer Index (BI) was developed based on an analysis of UCR reports while TTI in this analysis is a modified Akcelik equation which will not go infinity when V/C goes greater than 1.0. VD(R)F was calibrated based on VMT on subarea region and then validated on the corridor level. The performance of the VD(R)F was compared with the current VDF which is based on BPR in terms of reducing errors in count, speed and travel time estimation in corridors and links. Based on validation test set of corridor links (e.g. I-90, I-93), it was found that VD(R)F matches better with observed data rather than BPR. The VD(R)F increases the usefulness of traffic assignment and it works well with V/C over 1.0 compared to the BPR. Also, VD(R)F requires fewer iterations to reach equilibrium than the BPR. The performance also was studied on route choice and was found that VD(R)F may represent better traffic detour compared to the BPR.