Evaluation of Safety Strategies at Signalized Intersections (2011)

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16 The follow-up interviews discussed in the previous chapter revealed many problems with using data from many of the agencies. Some of the difficulties uncovered during the follow-up interviews included: • Lack of resources to participate in an evaluation. • Unable or unwilling to participate in this study. • Inadequacies in crash data. – No electronic crash data (i.e., crash reports are only available in paper copies). – Recent historical data (i.e., older than 3 years) are not available. – Crash data cannot be linked to intersections. • Installation records for the strategy were not available. • The strategy was installed at the same time the road was built (therefore, there is no before data for a before-after evaluation). • Lack of data for reference group intersections. • Inadequacies in supporting data. – No volume data. – No historical volume data. – No inventory data. • Other improvements were made at the same time. • Changes have been made since installation. Based on the outcomes of the interviews, a prioritized list of strategies for evaluation along with a work plan was submitted to the NCHRP Panel as part of the Interim Report. In devel- oping the list of prioritized strategies, tried, experimental, and proven strategies (based on NCHRP Report 500, Volume 12) were considered. The original solicitation had indicated that the focus should be on tried and experimental strategies. However, comments from the panel members on the quar- terly progress reports indicated that proven strategies should be considered as well. The prioritized list of strategies was developed after con- sidering the following: • Extent of the coverage in previous/ongoing work. Task 1 (literature review) indicated if a particular strategy has been rigorously evaluated by previous work or is being evaluated in another ongoing study, and the quality and predictive certainty of the CMFs developed so far. • Importance to the user. In order for the results of this study to be successfully implemented, it needs to focus on strate- gies that are important to the end user. Results of Task 2 (i.e., the importance as identified by the survey respondents) were used to determine the importance of the strategies to the end user. • Data assessment. The objective was to identify databases and determine the quality and quantity of data available for the development of high-quality CMFs using a rigorous before/after study. Phone interviews were conducted of respondents who potentially had data as part of Task 2 in order to make a preliminary assessment of this factor. The intent was to identify agencies that can provide data for multiple treatments and thereby reduce data collection costs. • Ability to identify crash effects. Strategies that may lead to diversion of traffic and/or have a system-wide effect would require more extensive and cost-prohibitive data collection efforts. Similarly, treatments that may have a small effect on total crashes may require a relatively large sample of sites to conduct a cost-effective evaluation. Based on these criteria and on the available budget, three treatments were identified for evaluation: • Adding protection for left-turning vehicles which includes the following treatments: – Introduce protected left-turn phasing. – Introduce permissive-protected or protected-permissive left-turn phasing. – Modify left-turn phasing and add left-turn lane at the same time. – Lengthen left-turn lanes. C H A P T E R 4 Prioritization of Strategies

17 • Optimize clearance intervals. • Install dynamic advance warning flashers. Work plans were developed for each of these. Note that this in effect includes six different treatments, since the first group includes four treatments. In addition to these six treatments, the following five treat- ments were identified as backups in case it was not possible to evaluate any of the six high priority ones: • Implement Split phasing. • Add left-turn lanes. • Provide or improve right-turn channelization. • Add signal heads. • Converting signalized intersections to roundabouts. The work plan consisted of a plan for the following four activities: • Collecting in-depth installation data. • Developing an experimental design for the treatments. • Collecting crash, roadway inventory, site characteristics. • Conducting a rigorous evaluation of the strategies. Interim Meeting with NCHRP Panel The prioritized list of strategies and the work plan were discussed at the Interim Meeting in Washington, D.C., in November 2007. At the meeting, the project team and the NCHRP Panel agreed on a set of Tier I treatments (higher pri- ority) and a set of Tier II treatments (lower priority) for the evaluation. At the meeting, it was agreed that funds would probably not be available to address all the treatments in Tier I. In addition, if data were not available to evaluate one or more of the treatments in Tier I, treatments in Tier II could be included. The following is a list of treatments in Tier I and Tier II. Tier I (higher priority): • Change to Protected phasing, • Change to Protected-permissive phasing, • Modify phasing and add left-turn lane, • Add left turn only, • Lengthen left-turn lane, • Dynamic advance warning flashers, • Optimize clearance intervals, and • Flashing yellow arrow. Tier II (lower priority): • Convert signalized intersections to roundabouts, • Right-turn channelization, • Add signal heads, • Improving friction at approaches to intersections, • Changing fonts, and • Change to Split phasing. After the interim meeting, the project team started compil- ing the necessary data for evaluating the treatments in Tier I. The project team could not find sufficient data for some of the treatments in Tier I, and after discussing this issue with the panel, it was decided to include one of the treatments from Tier II (convert signalized intersections to roundabouts) for which there appeared to be a research need coupled with the availability of data for a substantial number of treatment sites. The following is the final list of treatments that were evaluated in this study: • Install Dynamic Advanced Warning Flashers, • Convert Signalized Intersections to Roundabouts, • Change Clearance Intervals, • Change Left-Turn Phasing, and • Introduction of Flashing Yellow Arrow.