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7 CHAPTER 2 Literature Review Provided in this chapter is a summary of the literature High--The CMF was developed in a rigorous observational review and the identification of future and ongoing research. before-after study that incorporates what are currently con- Given that the focus of this effort was on developing CMFs, sidered the best study design and statistical analysis methods, the initial screening criterion applied to each study was namely the empirical Bayes (EB) method described by Hauer that the results must be founded on a crash-based analysis (as (1997) or the full Bayes (FB) method. opposed to analyses of driver behaviors, citations, or other Med-High--The CMF was developed in a before-after study "surrogate" measures). Hence, the focus of this review was on that incorporated sound (but not the current state-of-the-art) studies that evaluated treatments at signalized intersections statistical methods and/or may not have been reviewed using crash-based analysis. The studies for each treatment and "vetted" by an expert panel of researchers (as were the were further screened to determine which ones included the CMFs in the Highway Safety Manual). This level would also development of CMFs or a methodology that may be used to include CMFs that result from the combination of findings develop CMFs. The studies meeting this criterion and believed from different (i.e., less controlled) before-after and cross- to be the most credible were then subjected to a more critical sectional studies by an expert research panel. The panel's review. Each critical review was undertaken with the follow- judgment concerning the certainty level of the CMF would ing objectives: be reflected in our rating. This level would also include CMFs that have been developed in a rigorous meta-analysis by a recognized meta-analysis expert. (Meta-analysis is the 1. Evaluate the research approach and statistical methodology, combination of the results of various studies using tech- including an investigation of the potential for pitfalls such niques that allow the expert to accommodate some of the as regression-to-the-mean (RTM) or site-selection bias. shortcomings of the original research.) A thorough discussion of these possible pitfalls is presented Low-Med--The CMF was developed from cross-sectional in NCHRP Synthesis 295: Statistical Methods in Highway analysis (controlling for other factors statistically), or less- Safety Analysis (Persaud, 2001). than-rigorous before-after studies still judged to be of value 2. Document the magnitude and assess the confidence level (e.g., a before-after study in which regression-to-the-mean of any CMFs produced. was not viewed as a major potential bias due to the fact that "high-crash locations" were not selected for the treatment The results of the critical reviews are organized by strategies of the evaluation). listed from NCHRP Report 500, Volume 12. One of the out- Low--The CMF was developed in a simple before-after comes from the critical reviews was the confidence level of study without control for regression to the mean and other the CMFs of each treatment. This qualitative measure reflects confounders, or from cross-sectional studies where model- the level of predictive certainty in the CMF derived and is ing techniques and assumptions are questionable. a reflection of the study methodology. The confidence lev- els and the levels of predictive certainty can be qualified as Table 2.1 shows the predictive certainty for each treatment, follows: along with the key reference.

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Table 2.1. Key references for strategies/treatments from NCHRP Report 500, Volume 12, along with level of predictive certainty. Strategy Name P, T, or E1 Predictive Key References Certainty 17.2 A1: Employ multiphase signal operation protected left-turn P Medium-High Harkey et al., 2008. signal phase 17.2 A1: Employ multiphase signal operation permissive- P? Medium-High Lyon et al., 2005. protected or protected-permissive left-turn signal phase 17.2 A1: Employ multiphase signal operation split phases T Non-Existent No key studies 17.2 A2: Optimize clearance intervals P Medium-High Retting et al., 2002 17.2 A2: All red clearance interval P Low-Medium Souleyrette et al., 2004 Polanis, 2002 17.2 A3: Restrict or eliminate turning maneuvers using T Non-Existent No key studies channelization or signing 17.2 A3: Introduce/Prohibit RTOR T Medium-High Harkey et al., 2008 17.2 A4: Employ signal coordination P Non-Existent The following studies use surrogate measures (which are not yet proven) to try to deduce the effect on safety. Rakha et al., 2000 Berg et al., 1986 17.2 A5: Employ emergency vehicle preemption P Non-Existent No key studies 17.2 A6: Improve operation of pedestrian and bicycle facilities at Combination Low for Pedestrian Zegeer et al., 1982 signalized intersections: of Signals Elvik and Vaa, 2004 Pedestrian signs, signals, and markings P and T Crossing guards for school children Lights in crosswalks in school zones Pedestrian-only phase or pedestrian-lead phase during signal operation 17.2 A6: Improve operation of pedestrian and bicycle facilities at Combination Low-Medium Preusser et al., 1982. signalized intersections: Prohibition of RTOR of P and T 17.2 A7: Remove unwarranted signals P High Persaud et al., 1997. 1 P (proven), T (Tried), and E (Experimental); From NCHRP Report 500, Volume 12.

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Strategy Name P, T, or E1 Predictive Key References Certainty 17.2 B1: Provide or improve left-turn channelization: Providing Combination High Harwood et al., 2002. left-turn lanes of P and T 17.2 B1: Provide or improve left-turn channelization: Lengthening Combination Low Harwood et al., 2002. left-turn lanes of P and T 17.2 B1: Provide or improve left-turn channelization: Providing Combination High Khattak et al., 2004. left-turn lanes: Providing positive offset for left-turn lanes of Persaud et al., 2009. P and T 17.2 B1: Provide or improve left-turn channelization: Providing Combination Non-Existent No Key Studies positive guidance with channelization of P and T 17.2 B1: Provide or improve left-turn channelization: Delineating Combination Non-Existent No Key Studies turn path of P and T 17.2 B2: Provide or improve right-turn channelization: add High Harwood et al., 2002. exclusive right-turn lane P 17.2 B2: Provide or improve right-turn channelization: provide P Non-Existent The following two studies may evaluate the safety channelization that includes raised or painted islands aspects of different types of channelization treatments: NCHRP Project 3-78 (ongoing) NCHRP Project 3-89 (ongoing) 17.2 B2: Lengthen right-turn lanes P Non-Existent No Key Studies 17.2 B3: Improve geometry of pedestrian and bicycle facilities: Combination Non-Existent The following studies the safety effects of marked Continuous sidewalks of versus unmarked crosswalks at unsignalized Signed and marked crosswalks P and T locations. It is not clear if the results are Sidewalk set-backs transferable to signalized locations: Median refuge areas Zegeer et al., 2001 Pedestrian overpasses Intersection lighting Physical barriers to restrict pedestrian crossing maneuvers Relocation of transit stops Other traffic calming applications 17.2 B4: Revise geometry of complex intersections convert a T Non-Existent The following study did a meta-analysis based on four-leg intersection to two T intersections 9 studies that had looked at the effects of (continued on next page) 1 P (proven), T (Tried), and E (Experimental); From NCHRP Report 500, Volume 12.

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Table 2.1. (Continued). Strategy Name P, T, or E1 Predictive Key References Certainty converting four-leg to two T intersections. However, the study does not report whether these intersections are signalized or not. Elvik and Vaa, 2004. 17.2 B4: Revise geometry of complex intersections convert two T Non-Existent No key studies T intersections to one four-leg intersection 17.2 B4: Revise geometry of complex intersections improve P Non-Existent No key studies intersection skew angle 17.2 B4: Revise geometry of complex intersections Remove T Non-Existent No key studies deflection in through-vehicle travel path 17.2 B4: Revise geometry of complex intersections Close T Non-Existent No key studies intersection leg 17.2 B5: Construct special solutions: provide indirect left turn T Non-Existent No key studies 17.2 B5: Construct special solutions: Convert to roundabout T Medium High Persaud et al., 2001. Rodegerdts et al., 2007. 17.2 B5: Construct special solutions: Convert two-way streets to a T Non-Existent The following studies report reduction in one-way pair pedestrian crashes, but no information is available about the methodology that was used: Wiley, 1959 Karagheuzoff, 1972 17.2 B5: Construct special solutions: Construct interchange or T Non-Existent The following study did a meta-analysis based on grade separation 4 studies. However, none of these studies were from the USA: Elvik and Vaa, 2004 17.2 C1: Clear sight triangles T Non- Existent No key studies 17.2 C2: Redesign intersection approaches P Non- Existent No key studies 17.2 D1: Improve visibility of intersections on approach(es) T Non-Existent No key studies Improve signing and delineation Install larger signs 17.2 D1: Improve visibility of intersections on approach(es): T Low-Medium Lipinski and Wortman, 1976 Provide intersection lighting Preston and Schoenecker, 1999. Walker and Roberts, 1976. Donnell et al., 2009. Harkey et al., 2008. 1 P (proven), T (Tried), and E (Experimental); From NCHRP Report 500, Volume 12.

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Strategy Name P, T, or E1 Predictive Key References Certainty 17.2 D1: Improve visibility of intersections on approach(es) T Non-Existent No key studies Install rumble strips on approaches Install queue detection systems Install red-light hold systems 17.2 D1: Improve visibility of intersections on approach(es): Install T Low Sayed et al., 1999. dynamic advance-warnings flashers `Red Signal Ahead' 17.2 D2: Improve visibility of signals and signs at intersections: T Medium High Harkey et al., 2008. Install additional signal heads 17.2 D2: Improve visibility of signals and signs at intersections: T Medium High for The following study conducted a before-after Provide visors to shade signal heads Signal Lens EB evaluation using a combination of different Provide louvers, visors, or special lenses so drivers are Upgrade treatments to improve visibility. Different able to view signals only for their approach groups of intersections had a slightly different Install backplates set of treatments. However, results were not disaggregated by type of treatment: Sayed et al., 2007. 17.2 D2: Improve visibility of signals and signs at intersections: T High Sayed et al., 1998. Install larger (12 inch) signal lenses Harkey et al., 2008. 17.2 D2: Improve visibility of signals and signs at intersections: T Non-Existent No key studies Remove or relocate unnecessary signs Provide far-side left-turn signal 17.2 E1: Provide public information and education T Non-Existent No key studies 17.2 E2: Provide targeted conventional enforcement of traffic laws T Non-Existent No key studies 17.2 E3: Implement automated enforcement of red-light running P High Council et al., 2005. Shin and Washington, 2006 Miller et al., 2006. 17.2 E4: Implement automated enforcement of approach speeds T Non-Existent The following studies looked at the effect of automated enforcement on safety at different corridors, but did not report on crashes at signalized intersections: Cunningham et al., 2005. Chen et al., 2002. Mountain et al., 2004. (continued on next page) 1 P (proven), T (Tried), and E (Experimental); From NCHRP Report 500, Volume 12.

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Table 2.1. (Continued). Strategy Name P, T, or E1 Predictive Key References Certainty 17.2 E5: Control speed on approaches: E Non-Existent No key studies Construct a horizontal curve to reduce speeds Speeding vehicle activated traffic signals Traffic calming treatments 17.2 F1: Restrict access to properties using driveway closures or T Low Xu, 2001 turn restrictions 17.2 F2: Restrict cross-median access near intersections T Non-Existent No key studies 17.2 G1: Improve drainage in intersection and on approaches T Non-Existent No key studies 17.2 G2: Provide skid resistance in intersection and on approaches T High Harkey et al., 2008. 17.2 G3: Coordinate closely spaced signals near at-grade railroad T Non-Existent No key studies crossings 17.2 G4: Relocate signal hardware out of clear zone T Non-Existent No key studies 17.2 G5: Restrict or eliminate parking on intersection approaches P Non-Existent The following study did a meta-analysis based on 13 studies that had looked at the effects of changes in parking. However, it is not clear how many of the locations were close to signalized intersections: Elvik and Vaa, 2004. 1 P (proven), T (Tried), and E (Experimental); From NCHRP Report 500, Volume 12.