The National Academies

NCHRP 08-143 [Pending]

Guide to the Application of Spatial Segmentation on Travel Time Reliability Measures

  Project Data
Funds: $150,000
Contract Time: 24 months
Staff Responsibility: Jennifer L. Weeks


Travel time reliability measures create a consistent means of comparing the performance of different highways in the network. Travel time reliability measures inform decisions regarding highway capital and operating investments and facilitate accurate public reporting of highway travel times and performance. Additionally, travel time reliability is a required federal performance measure for state departments of transportation (DOTs) and large metropolitan planning organizations (MPOs).
Travel time reliability measures are affected by the way highway corridors are divided into segments for data collection, analysis, and reporting. Corridors with fewer segments may “average out” the impacts of congestion “hot spots” along a corridor, creating less reliability in practice. Shorter and more intentionally defined road segments may have the effect of creating more predictable and consistent reliability measures by isolating the impacts of places that experience more or less episodes of congestion. However, the travel time reliability measures calculated for longer corridors with many “hot spots” can be less certain and comparisons of reliability measures calculated for roads within a network cannot always be trusted.
Typically agencies obtain travel time and speed data from third-party probe data suppliers who use predetermined segments. To improve travel time reliability measures, practices, and reporting, transportation practitioners need direction on the application of methods for supplementing these data using segmentation and aggregation to produce the most consistently reliable measures, ones that can be compared across the network and used for public communication and investment decision-making.
The objective of this research is to develop a guide for designing, modeling, and applying roadway segmentation in travel time reliability measurement to generate improved reliable predictions across similar roadway contexts. At the minimum, the research shall:
1. Identify different segmentation and aggregation methods for measuring travel time reliability in use by state DOTs, MPOs, and probe vehicle data providers across the nation.
2. Evaluate the variability in travel time reliability measures using different segmentation or aggregation methods appropriate for application in different roadway networks and contexts, taking into consideration roadway type, local density and environment, and temporal and geographic contexts, as appropriate.
Proposers are asked to articulate a research plan comprised of a set of sequenced specific tasks that will be implemented through the proposed research to successfully achieve the research objective within the limits of the budget provided.
The proposed research plan should be divided into two logical phases with the first phase focused on data gathering and assessment and the second phase focused on production and validation of guidance and products for industry use. The plan shall (1) include a kick-off web conference with the NCHRP project panel to review the amplified work plan convened within 1 month of the contract’s execution; (2) identify specific tasks, activities, and analyses that will be needed to successfully achieve the research objective; and (3) incorporate ample opportunities for NCHRP panel review and engagement on key tasks and deliverables throughout the study process. 
The following offers a potential manner of dividing the work. Proposers shall articulate a detailed research plan with a clear approach to achieving the research objective that at a minimum incorporates these elements.
Task 1. Conduct a comprehensive literature review documenting the theory and practice of travel time reliability measurement by the transportation industry.
Task 2. Identify, synthesize, and document the current state of practice by state DOTs with respect to using different methods of segmentation to derive travel time reliability measures. Proposals should specify specific methods of data collection for the synthesis. It is expected that direct engagement of practitioners will be part of the data collection through such means as targeted surveys, interviews, or workshops with state DOTs and other practitioners.
Task 3. Analyze data and information collected in Tasks 1 and 2 for its practical application by the industry presented in a manner that easily distinguishes the effects of different roadway segmentation strategies to support different analytical and reporting purposes.
Task 4. Identify a short list of best practices and/or lessons learned from the industry for integration into the guide and other instructional products recommended by the research team in the subsequent phase of work.
Task 5. Prepare an interim report that documents the research process conducted and data collected in Phase I. The report shall contain a detailed Phase II work plan for identifying and testing candidate approaches to network segregation for different types of projects using the information collected in previous tasks. Lastly, the report should include an outline of the guide and detailed description of the other research products. The research team will meet with the project panel to discuss the interim report. 
Task 6. Identify existing candidate approaches to network segregation to support travel time reliability measurement for different types of roadway projects using the information and data collected in Phase I.
Task 7. Pilot test at least 3 of the segmentation approaches described in Task 6 using case study data and highway project examples. The case studies should demonstrate the impact of choices made on the length of segments, selection of segment breaks, and other segmentation issues on the reliability of travel time performance measures. The analysis should illustrate any issues experienced by the agencies in their application as well as measure the extent of impact on the travel time reliability measures that come from their use. 
Task 8. Prepare a guide and supplemental products identified in the approved Phase II work plan.

Products of this research shall at a minimum include:

1. A practical guide providing comprehensive instruction on the design and implementation of segmentation and aggregation to support a variety of travel time reliability measurement contexts;

2. A matrix or summary graphic documenting the benefits and impacts of distinct methods, both in use and/or proposed through this research, for assessing and documenting travel time reliability under different contexts;

3. A process diagram, graphic, or summary depiction of a recommended process for defining and using appropriate segments for the purposes of measuring travel time reliability for different purposes;

4. A research report documenting the research process, summarizing and presenting all data collected, methods used, analysis of methods, and resultant recommendations or guidelines for implementation by agencies;

5. An implementation plan specifying an approach to facilitating the application of the research by the transportation industry.

STATUS: Contract is pending

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