In This Case Study You Will Learn
- North Carolina utilized statewide probe-based travel time data to inform signal retiming process.
- The guiding principle for the revised prioritization framework and tool was tying metrics to actionable items.
- The web-based prioritization tool incorporates travel times, reliability, trends, safety performance, as well as annual average daily traffic (AADT) to provide visualizations and standard reports.
The North Carolina Department of Transportation (NCDOT) maintains over 380 closed loop signal systems comprising over 2,400 controllers, with an annual retiming program used to ensure the signal plans are current and effective. Traditionally, this retiming program centered on a 3-year update cycle for signal systems. The re-timing and prioritization process was driven by the centralized NCDOT Central Office for System Timing (COST) group, with input from the 14 Divisions across the state. However, the process was not performance-based with geographic dispersion of signals and data collection costs preventing the department’s ability to consider travel time and safety performance measures.
With the availability of statewide probe-based travel time data, the NCDOT embarked on a process to integrate statewide travel time, reliability, safety, and exposure measures into a centralized retiming prioritization database, and commissioned the development of a web-based tool to assist COST and the 14 NCDOT divisions to prioritize systems and select candidates for re-timing.
TSMO Planning, Strategies and Deployment
Two years ago, the Central Office System Timing (COST) section began testing probe travel time data for identification of systems in need of retiming with initial internal work showing consistent results with field collected travel times. ITRE and KAI developed a performance-based prioritization tool with similarities to the prioritization framework used for NCDOT’s STIP. The performance framework used cross-cutting performance measures, integrating congestion, reliability, safety, exposure, and historical data trends into the prioritization process.
The performance measures were based on industry best practice after outreach to peer agencies, including Pennsylvania DOT and others. The selected performance measures were integrated into an easy-to-use web-based platform, assuring ready data access and availability of measures for COST staff, NCDOT divisions, as well as consultants engaged in the re-timing efforts. The new data-driven prioritization process provides transparency for public and private stakeholders in NCDOT’s retiming process, and allow consideration of continuous data trends, as opposed to one-time snapshots of data obtained from traditional travel time data collection.
In order to test the viability of the selected performance measures, the team explored historical performance trends and assessed if the measures were sensitive to prior re-timing efforts. The results provided NCDOT and stakeholders confidence that selecting corridors based on the defined measures would assure that selected corridors would show a high likelihood of benefiting from an investment in re-timing.
The guiding principle for the revised prioritization framework and tool was Tying Metrics to Actionable Items. For example, a corridor with steady increases in average and 95th percentile travel times with demands exceeding capacity in the peak hour may not be a candidate for re-timing but may instead require capital improvements. But a corridor with sudden travel time changes due to new developments may be a prime candidate for retiming. Similarly, a corridor with poor performance around edge of peak periods may benefit from re-timing and adjustments of time-of-day plans. NCDOT’s goal with this effort was to maximize the use of limited re-timing resources by targeting corridors with the greatest potential for benefiting from the retiming investment.
Communications Planning and Execution
While originating from NCDOT’s COST group, a multi-disciplinary core project stakeholder group was assembled to shape the project vision, provide continuous feedback in the development process, and beta-test tool deliverables. Core stakeholders from within NCDOT included: Central Office Signal Timing (COST), ITS, Signal Design, Safety Units, NCDOT Statewide Operations, and IT and computing services.
The project development team included academia and consultants, with the team from N.C. State University assuring that the database and tool web-hosting remained within a state agency. In addition to the core team, an extended stakeholder group was assembled to provide early feedback in the development process, test tool prototypes, and review results of the prioritization process. The extended stakeholder group included: Representatives from all 14 NCDOT divisions; and Stakeholders from five metropolitan areas across the state.
Outcome, Benefit and Learnings
The web-based prioritization tool incorporates travel times, reliability, trends, safety performance, as well as annual average daily traffic (AADT) to provide visualizations and standard reports that COST and the NCDOT Divisions can use to better inform the selection process.
Travel time performance measures include average and reliability metrics by time of day as well as the FHWA LOTTR metric, enabling the program to identify signal systems on the NHS that can be targeted for improving the statewide MAP-21 reliability performance measure. Safety data include critical crash rates, crash densities, and severities, while AADT data provide information on the number of vehicles served by each system. NCDOT plans to utilize the tool for prioritizing the FY2019 program and update the datasets for continued use in future years. All performance measures can be filtered by NCDOT division, as well as assessed on an individual corridor basis.
The NCDOT Corridor Signal Retiming Prioritization Tool has been completed and is currently being used to select corridors for retiming in the 2018/2019 fiscal year.
Operations Area of Practice
- Traffic Signal Timing Software
- Case Studies & Lessons Learned