CHAPTER 1

Background

Overview

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 crash modification factors (CMFs) for several ATSPM reports being used for monitoring and managing signalized intersections. Additionally, this project estimated the potential return on investment in ATSPM deployment based on consideration of both the safety and operational benefits of this deployment.

This report documents the research approach and results of NCHRP Project 17-109, Crash Modification Factors for Automated Traffic Signal Performance Measures. The report is organized as follows:

Research Approach

Background

NCHRP Project 17-109 was undertaken to assist DOTs and local transportation agencies quantify the safety benefits resulting from the deployment of ATSPM signal timing by developing a set of CMFs that predict the effect of a change in signal timing or operation on the change in crash frequency or severity.

Objective

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.

Research Tasks

The research tasks were planned to be conducted in three phases. A data collection plan and an analysis plan were developed in Phase I based on information obtained from the literature review and survey of targeted agencies. A methodology for evaluating the safety effect of ATSPM-based signal timing was developed in Phase II. The methodology was revised in Phase III based on two webinars with state DOT representatives. The task titles are identified in the following list.

Phase I – Planning

• Task 0. Prepare Amplified Work Plan
• Task 1. Review Literature and Conduct Survey
• Task 2. Identify Required Data
• Task 3. Identify Knowledge Gaps
• Task 4. Develop Data Collection Plan, Analysis Plan, and Implementation Roadmap
• Task 5. Prepare Interim Report No. 1

Phase II – Development of Methodology

• Task 6. Develop Methodology for Evaluating the Safety Effects of ATSPM-based Signal Timing
• Task 7. Prepare Interim Report No. 2

Phase III – Final Deliverables

• Task 8. Develop and Conduct Webinars
• Task 9. Revise Methodology and Related Materials
• Task 10. Prepare Final Deliverables

The key research products are the ATSPM Evaluation Methodology and the Implementation Roadmap. The ATSPM Evaluation Methodology was delivered as a stand-alone guidebook that describes a methodology for using the CMFs developed in this project. It includes a list of required data, procedural steps, and sample calculations.

The Implementation Roadmap describes a framework of goals, objectives, and strategies for implementing the research products in the American Association of State Highway and Transportation Officials (AASHTO) Highway Safety Manual (HSM) and the FHWA CMF Clearinghouse. The roadmap describes a series of implementation steps (and their associated timeline) that are intended to promote inclusion of Project 17-109’s CMFs in the HSM and Clearinghouse.

Overview of ATSPM Evaluation Methodology

The research team developed an evaluation methodology for the development of CMFs describing the safety effects of ATSPM-based signal timing and operation. The terms included in the methodology to categorize these CMFs are defined in this section. The researchers grouped the CMFs to be developed into two categories (i.e., cases) based on the spatial scope of the safety evaluation. These two CMF cases and their associated evaluation scopes are identified in the following list:

• “Case A CMFs” for overall evaluation of ATSPM-based signal systems, and
• “Case B CMFs” for site-based evaluation of individual ATSPM reports.

An overview of the evaluation methodology for cases A and B is provided in Table 1 and Table 2 respectively. The overview describes the spatial analysis scope, application scenarios, and analytic framework for each case. The proposed approach incorporates safety performance functions (SPFs) from the HSM and similar predictive models from the Highway Capacity Manual (HCM) into the ATSPM Evaluation Methodology, thereby leveraging the associated research such that the resources of Project 17-109 can be focused on the development of CMFs for a wide range of ATSPMs.

Table 1. Overview of the ATSPM Evaluation Methodology based on Case A CMFs.

Components

Spatial Analysis Scope: signal system

Example Application Scenario

Predict safety effect of conversion from traditional signal system management and monitoring approach to an ATSPM-based approach.

Analytic Framework Identify boundaries of signal system and obtain required data. Estimate the average crash frequency for the existing system Np,ex using its observed crash history. Select CMFA based on guidance in the ATSPM Evaluation Methodology guidebook. Compute average crash frequency for the converted system as: Np,alt = Np,ex × CMFA. Compare with Np,ex from Step 2.