Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 7
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.
OCR for page 8
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.
OCR for page 9
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.
OCR for page 10
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.
OCR for page 11
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.
OCR for page 12
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.
