Research Projects and Reports : Incorporating Variable Peak-to-Daily Ratios into FSUTMS to Reduce Assignment Errors

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Incorporating Variable Peak-to-Daily Ratios into FSUTMS to Reduce Assignment Errors (1.02MB)

The standard highway assignment model in the Florida Standard Urban Transportation Model Structure (FSUTMS) is based on the equilibrium trip assignment method. The method involves running several iterations of all-or-nothing capacity-restraint assignment with an adjustment for travel time to reflect delays encountered in the associated iteration.

This iterative link time adjustment process is accomplished through the widely used Bureau of Public Roads (BPR) volume-delay equation. While the output assignments from traditional forecasting models are usually given in daily traffic, the input capacities are typically given in hourly traffic. A peakto- daily ratio (CONFAC) is computed as the highest hourly volume of a day divided by the total daily volume. Although the current version of FSUTMS allows the use of different CONFACs for indiviudal facility types, it still assumes that all roadways belonging to a specific type have the same peaking characteristics. The objective of this research is to improve the accuracy of trip assignments in FSUTMS by: (1) calibrating the relationships between CONFAC and a congestion measure for each facility type, using traffic count data, and (2) applying the calibrated relationships in FSUTMS' trip assignment. It was found that the output accuracy between constant and variable CONFAC assignments was not significantly different and the desired improvement in assignment results of the variable CONFAC model was not empirically evident.

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