Synthesis of Safety for Traffic Operations - Transports Canada

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Synthesis of Safety for Traffic Operations March 2003 CHAPTER 4: TRAFFIC SIGNS Signing General Lyles et al (1986) Lyles et al (1986) examined the safety effects of jurisdiction-wide upgrades to traffic control devices in Michigan. Despite the broader title of “traffic control devices”, only upgrades to traffic signs were included in the treatment. Jurisdictions where the sign upgrades took place varied in size. The study methodology was a before-after study with a modified control group. Treatment locations were all local streets; the control group consisted of numbered state routes within the same jurisdiction. It is recognized by the researchers that the state routes differ in many respects from local streets. However, it was thought that using a control group from the same jurisdiction of the treatment locations would better control for confounding by weather, traffic volume changes, and other local factors. The researchers used crash frequency, severity, and the distribution of crash types as the measures of effectiveness. Three years of before, and three years of after crash data were used. The CMF for sign upgrades on the local street system is 1.04 (see Table 4.1). TABLE 4.1: Safety Impacts of Traffic Sign Upgrades in Michigan Site Three-year Crash Count Before After CMF Treatment 3718 3523 0.95 Control 1753 1593 0.91 Adjusted CMF for treated locations 1.04 Lyles et al conclude that there is no evidence to suggest that jurisdiction-wide sign upgrading on local streets has any effect on safety (both crash occurrence, and severity). The researchers acknowledge that a jurisdiction-wide analysis of sign changes which in most cases were minor (ex., minor adjustments to sign placement) is likely inappropriate. The expected safety benefits of sign upgrades are small, and the number of potential confounding factors present and unaccounted for in a jurisdiction-wide analysis overwhelm the results. Tople (1998) Tople (1998) in an evaluation of hazard elimination and safety projects included a review of the safety benefits of traffic signing. The documentation available does not indicate what type of signing was implemented. In any event, the evaluation was a naïve beforeafter study of crash frequency and crash severity. The impact on crash severity was Page 61
Traffic Signs determined through a comparison of equivalent property damage only crashes, using monetary conversations deemed appropriate by the investigation team. Three years of before and three years of after crash data were used in the analysis. The results of the analysis are presented in Table 4.2. TABLE 4.2: Safety Impacts of Traffic Signing in South Dakota No. Crashes EPDO Crashes* Improvement of Type Before After CMF Before After CMF Sites Traffic 6 1403 1330 0.95 48860 41588 0.85 Signing * EPDO crashes were calculated as (1300*F)+(90*I)+(18*N)+(9.5*P)+PDO where: F = fatal crash I = incapacitating injury crash N = non-incapacitating injury crash P = possible injury crash PDO = Property damage only crash The Tople analysis possesses many potentially serious flaws. Most importantly, the sites were selected for treatment as part of a safety program. This means that the crash record was likely abnormally high, and there is a great potential for regression to the mean artefacts. This shortcoming is likely offset somewhat by a failure to account for changes in exposure. Traffic volumes were not controlled for, but typically volumes tend to increase which would lead to a higher “after” count of crashes. In the end, the South Dakota results are based on a limited number of sites and weak analyses. Transport Research Laboratory (2000) The TRL (2000) of the United Kingdom has collected information on the safety impacts of signing through the MOLASSES database (see the section on “Signalization” for more information on MOLASSES). Specific information on whether “signing” means to incorporate new signs, improved signing, or some combination on both is not provided. Therefore, the results are generalized and are only adequate to provide cursory guidance on the magnitude of the potential for safety improvement. The results are shown in Table 4.3. TABLE 4.3: Safety Effects of Signing Modifications in the United Kingdom Number of Number of Crashes Setting Locations Before After CMF Urban 222 1536 1044 0.68 Rural 136 879 521 0.59 Page 62
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Transport Canada Transports Canada
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Synthesis Of Safety For Traffic Ope
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DISCLAIMER The material presented h
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EXECUTIVE SUMMARY Traffic operation
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Synthesis of Safety for Traffic Ope
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TABLE OF CONTENTS DISCLAIMER REJET
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LIST OF TABLES TABLE 3.1: Crash Rat
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Chapter 1: Chapter 1: Introductio I
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Introduction Before proceeding, a f
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Introduction Literature regarding t
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Introduction The current science of
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Introduction adequate size control
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Introduction In concluding this sec
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Page 79 and 80: Intersection Control The CMFs are 0
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Page 83 and 84: Intersection Control Shebeeb (1995)
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Page 105 and 106: Pavement Markings The study methodo
Page 107 and 108: Pavement Markings Hall (1987) Hall
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Chapter 9: Chapter 9: Turn Lanes Tu
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Turn Lanes TABLE 9.2: Safety Impact
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Turn Lanes Vogt determined that a l
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Turn Lanes 1998 one of their high c
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Turn Lanes Section TABLE 9.10: Safe
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Turn Lanes A T = ADT 0.91 L 0.852 e
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Turn Lanes CMF = 1 - 0.7 P lt/d 0.0
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Turn Lanes occurred within 250 feet
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TABLE 10.3: Speed Changes Resulting
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Sites Where Applied 1 URBAN Island
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Forbes and Gill (2000) undertook a
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Ideally, three years of before and
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Kermit and Hein (1962) looked into
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The effects of the transverse pavem
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Chapter 11: Chapter 11: Other Other
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Other Safety Issues collisions per
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Other Safety Issues THIS PAGE IS IN
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References References
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References Bloch SA (1998) “Compa
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References Hamilton Associates (199
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References Lee JT, Maleck TL, and T
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References Polanis SF (1998) “Do
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References Vancouver, City of (1999
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References THIS PAGE IS INTENTIONAL
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Appendix B: Critical R Appendix B:
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Critical Reviews Worksheet for Eval
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Critical Reviews 2. Are the expecte
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Appendix D: Appendix D: How to Use
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How to Use Safety Performance Funct
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How to Use Safety Performance Funct
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Synthesis of Safety for Traffic Operations - Transports Canada

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