Summary

Introduction

This report documents the research approach and results of NCHRP Project 17-109, “Crash Modification Factors for Automated Traffic Signal Performance Measures.” Automated traffic signal performance measures (ATSPMs) offer an alternative approach to traditional signal timing. The objectives of the project were: (1) to develop crash modification factors (CMFs) for ATSPM-based signal timing for various conflict types and levels of severity, and (2) to estimate the potential return on investment in ATSPM deployment based on consideration of both the safety and operational benefits of this deployment.

In addition to the report, the project produced the following documents to facilitate implementation of the developed products and research outcomes:

• ATSPM Evaluation Methodology, a guidebook that describes the methodology developed for using the CMFs to evaluate the safety effects of ATSPM-based signal timing,
• Application Spreadsheet, which automates the CMF calculations described in the ATSPM Evaluation Methodology,
• A stand-alone Technical Brief summarizing the research results,
• Implementation Roadmap, which describes strategies and tactics for promoting the research products in the Highway Safety Manual (HSM) or CMF Clearinghouse, published in this document as Appendix A, and
• An Implementation of Research Findings and Products technical memorandum, published in this document as Appendix B.

Background and Motivation

Automated traffic signal performance measures (ATSPMs) have been shown to be a promising tool for proactively monitoring and managing signalized intersections. The use of ATSPMs to improve operations has been thoroughly researched and is generally well-established in the industry. In contrast, the use of ATSPMs to improve safety has not been as well researched. To overcome this knowledge gap, NCHRP Project 17-109 developed CMFs for several ATSPM reports being used for monitoring and managing signalized intersections.

Based on the state of the practice review and feedback from agencies, the research team recognized that practitioners desire CMFs that can be used for the following two ATSPM-based safety evaluation cases:

• Case A CMF for overall evaluation of ATSPM-based signal systems. The case A CMFs focus on an arterial system and estimate the safety effect of transitioning from traditional signal timing/monitoring approach to an ATSPM-based approach. As a result, instead of focusing on the effect of a specific ATSPM report for an individual intersection (or an individual intersection movement), the case A CMFs focus on the arterial system that includes both travel directions and side streets.
• Case B CMF for site-based evaluation of individual intersection using one or more specified ATSPM report. Each case B CMFs focus on a specific ATSPM report and computes the safety effect of a reported change in signal timing or operation (e.g., the safety effect of a change in platoon ratio because of improved signal coordination using ATSPM reports).

Research Approach

Prioritization of Knowledge Gaps

There are approximately 30 ATSPM reports that can be utilized by agencies to operate and maintain traffic signals. Therefore, the research team needed to prioritize the ATSPM reports for the development of case B CMFs. Based on the findings from the literature and agency outreach, the research team prioritized knowledge gaps based on the potential safety impact of ATSPM reports, availability of data/sites to develop CMFs, and practitioner’s interest in CMFs for specific ATSPM reports. Based on the prioritization framework, the research team developed five study designs for CMF development, as listed below.

• Case A1. Use of ATSPMs to Manage a Signal System
• Case B1. Percent Arrivals on Green (typically obtained through the Purdue Coordination Diagram ATSPM Report)
• Case B2. Yellow and Red Actuations (typically obtained through the Yellow and Red Actuations ATSPM Report)
• Case B3. Split Failure (typically obtained through the Purdue Split Failure ATSPM Report)
• Case B4. Left-Turn Gap Analysis (obtained through the Left-Turn Gap Analysis ATPSM Report)

Data Collection and Study Method Summary

For the case A CMF development, the research team collected data for six arterial street study sites (two arterial streets each from three different states) where ATSPMs are in operation. These states included Virginia, Georgia, and Utah. For each arterial street, the team collected annual average daily traffic (AADT) volume data, posted speed limit, number of through lanes, median type, and presence of a left turn bay. A before-and-after study method was used to quantify the CMFs for each arterial site. Data was also collected for a set of comparison signal systems that do not have ATSPM deployment. The comparison sites were used to account for changes that occur during the before-after time periods that were not due to ATSPM deployment. Thus, the comparison sites were used to compute a CMF that reflects only the change in safety due to ATSPM deployment. The data time period included at least three consecutive years of data for the “before” period and at least one year of data for the “after” period. The data-time-period coincided with the most recent years for which crash data are available. For each site, the research team computed various types of CMFs including CMFs by crash severity, by facility type, by traffic time periods, and all severities, facility types, and time periods combined.

For the case B CMF development, the research team collected data for several signalized intersection study sites where ATSPMs are in operation. For each intersection, the team collected AADT volume data and data describing various intersection characteristics. Additionally, the team obtained archival high-resolution signal controller data and detector mapping data for each intersection. The data was collected from three different states and from a total of 42 signalized intersections 109 intersection legs. For each intersection, archival ATSPM data was obtained for a minimum of two years and a maximum of three years. To develop the case B CMFs, a cross-sectional study method was used. The assembled data was used to infer several CMFs using regression analysis of the yearly observations in the database.

Research Findings

Case A CMF Results

The research team computed various CMFs for each of the six ATSPM-operated arterial street study sites. The overall CMF values indicated a wide range in safety effects (including both reductions and increases in crashes) when all severities, all facility types, and all time periods are considered. Overall crash

frequency decreased by 20 percent, 7 percent, and 1 percent at site 1, site 2, and site 3 respectively. In contrast, overall crash frequency increased by 22 percent, 33 percent, 34 percent at site 4, 5, and 6, respectively. For fatal-and-injury (FI) crashes and property-damage-only (PDO) crashes, site-to-site variation was similar to those for overall crashes, although the CMF values for FI crashes were in general lower than those for PDO crashes.

The computed CMF values suggest that signal systems operated using ATSPM could be associated with a reduction in crash frequency and severity for some conditions. However, the values also suggest that ATSPM operation could be associated with an increase in crash frequency and severity for other conditions. It is likely that the safety outcomes realized by an agency implementing ATSPMs will reflect how the signal system is operated. For example, changes to the signal operation that result in a change in (a) operating speed, (b) percentage of vehicles stopping, or (c) cycle length (and related phase termination frequency) could be associated with a change in safety. The findings of this research indicate that there is a potential safety benefit of ATSPM operation; however, the agency should monitor the system after ATSPM implementation (using both safety surrogates and crash history) to ensure that there has been no degradation in safety.

Case B CMF Results

The research team developed one pair of CMFs for each of the following four ATSPM-report-based measures: platoon ratio, split failure, vehicle entry to intersection during the yellow indication (i.e., yellow actuation), and left-turn gap availability. For each of the first three measures listed, one CMF quantified the measure’s relationship with FI crash frequency and one CMF quantified the relationship with PDO crash frequency. For the last measure listed, one CMF quantified the relationship with left-turn-with-opposing-through-vehicles (LT) crashes. The other CMF quantified the relationship with non-LT crashes. Each of these CMFs was developed as an equation wherein the CMF value is computed as a function of relevant intersection characteristics.

The platoon ratio CMFs indicate that intersection approaches with a higher platoon ratio (i.e., with favorable progression) have a lower crash risk than those approaches with unfavorable progression. Estimated CMF values suggested that an increase in platoon ratio (i.e., improved progression) would reduce both fatal-and-injury (FI) crashes and property damage only (PDO) crashes, where the reduction in crash risk would be more pronounced with fatal-and-injury crashes.

The split failure CMFs indicate that intersection approaches with a higher probability of split failure have a higher crash risk. For a given probability of split failure, results indicated that crash risk is lower for those intersection legs with a longer cycle length. The researchers believe that this trend occurs because, for a given probability of split failure, longer cycles mean fewer cycles in each hour, and thus fewer number of split failures per hour. Unlike the platoon ratio metric; however, the change in FI crashes and PDO crashes as the probability of split failure changes were relatively similar.

The yellow actuation CMFs indicate that for a given approach volume, intersection approaches with a higher portion of the approach volume entering during yellow have a higher crash risk. For a given probability of yellow entry, crash risk is shown to be higher for those approaches with higher volume. The relationship between vehicle entry during the red clearance interval was investigated but none of the available study sites had a detection design that was able to reliably quantify red entry frequency. Due to these challenges, no CMF was developed for red actuations.

Finally, for the left-turn gap availability CMF, the results indicated that intersection approaches with high-left turn permissive capacity (which increases as the left-turn gap availability increases in the opposing through lane) have a lower crash risk than those approaches with a low capacity. Results also showed that for a given left-turn permissive capacity, approaches with a high speed limit have a higher crash risk than those with a low speed limit.

Suggestions for Future Research

Additional research is suggested to address knowledge gaps that were not addressed during this project, as described below:

• This project’s data collection effort for case A CMFs included six ATSPM-operated arterials. The results obtained from the case A CMF analysis suggested a wide variation in safety effects of ATSPM-operated arterials among arterials and among states. To better understand the relationship between ATSPM-operated arterials and their safety effects, and develop a more consistent CMF value, additional research is suggested that would expand the number of ATSPM-operated arterials that would be amenable to a before-after study for the purpose of quantifying a CMF that would describe the change in safety associated with ATSPM deployment.
• For case B CMFs, the exploratory analysis of red actuations (i.e., entry during red interval) found that an increase in the proportion of approach volume entering during the red interval was associated with a reduction in crash rate. This finding is contrary to the results reported in the literature. It is partly explained by the researchers’ selection of study sites that were ultimately found to be suboptimal for the examination of red actuations. Specifically, they unknowingly selected intersections with detection designs that were unable to reliably screen out right-turn-on-red maneuvers. To address this challenge, it is suggested that future research be undertaken to develop a CMF for red actuations using intersections having detection and monitoring equipment that can reliably exclude right-turn-on-red vehicles from the entry-on-red counts.
• The research prioritized existing ATSPM reports for the development of case B CMFs. This prioritization process led to the development of case B CMFs for arrivals on green (platoon ratio), split failures, yellow actuations, and left-turn gap analysis. However, in addition to these four ATSPMs, there are other ATSPM reports (and related measures) that could have a quantifiable relationship with crash frequency or severity. It is suggested that research be undertaken to explore the relationship between other ATSPM reports (that likely influence safety) and crash frequency and severity.