The National Academies

NCHRP 08-143 [Anticipated]

Impact of Spatial Segmentation on Travel Time Reliability Performance Measures

  Project Data
Funds: $150,000
Staff Responsibility: Lawrence D. Goldstein
Fiscal Year: 2021

This project has been tentatively selected and a project statement (request for proposals) is expected in August 2020. The project statement will be available on this site. The problem statement below will be the starting point for a panel of experts to develop the project statement.

It can be shown that measures of variability in corridor travel time, unlike average values, depend in part on how that corridor is broken up into segments for measurement.  This affects all agencies that use such metrics in performance measurement and/or project selection, especially all state DOTs and large MPOs subject to the requirements of performance management spelled out in such federal-aid surface transportation requirements as the MAP-21 Act.  The proposed research will develop a series of guidelines and best practices suitable for implementation by transportation agencies.  Step-by-step processes will be developed, and guidance for implementation (including analysis tools) will be provided.  

Only limited research has been conducted on this subject to date.  One recent report (cited on page 3) evaluated the impact of spatial segmentation on arterial system delay measures using several data sets from southeastern Virginia.  Compared to industry standard TMC road segments, custom segments based on engineering judgment (such as homogeneous traffic volume, speed limit, number of lanes, signal density, etc.) decreased network delay by −3.4% and reliability index measures by 0.7% to 1.9% (using GPS data from INRIX). Use of very long segments, such as the entire corridor in each direction, noticeably averaged out congestion and decreased the delay by 29% and reliability index measures by 2.3% to 4.9%. The corresponding reductions using the National Performance Measures Research Data Set (NPMRDS) were 43% for delay and 5.7% to 9.5% for reliability index measures.  This study was focused on arterial roads, and it is unclear if those results would translate to other areas or to freeway facilities.

The objective of this research is to determine the statistical implications of current methods for determining travel time and its reliability and propose a set of corresponding measures, including consideration for segmentation, that are suitable for roadway systems. A recommendation on how to compare travel time reliability among corridors, regions, or even states can facilitate communications of this measure and is an expected outcome from this project.  In carrying out this research, attention should be given to Federal Highway Administration work with the I-95 Corridor Coalition and work undertaken by the AASHTO Committee on Performance Based Management.

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