Synthesis of Safety for Traffic Operations - Transports Canada

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Synthesis of Safety for Traffic Operations March 2003 Horizontal Curve Signing and Marking Arnott (1985) Arnott (1985) undertook a safety evaluation of traffic activated curve speed warning signs in Ontario that were used at freeway interchanges. A total of five sites were evaluated; three had the signs located on the interchange ramp, two had the sign located on the mainline of the freeway. The treatment was essentially a warning sign that is supplemented by a “TOO FAST” tab sign that was illuminated when upstream detectors identified a vehicle exceeding a preset speed threshold. The actual configuration of the warning sign varied somewhat from location to location; the commonality between sites is the actuated “TOO FAST” warning. The study methodology was a naïve before-after evaluation of crash frequency and distribution. Nine to eleven years of crash data were available for each site, although the after periods at three of the sites were 13 months or less. Crashes that were coded as “speed too fast” under driver action were the target crashes. The results of the analysis are shown in Table 4.4. TABLE 4.4: Safety Impacts of Traffic Activated Curve Speed Warning Signs Site Period Proportion of Annual Total Target crashes that are Target Crashes Crashes target crashes Crashes (%) CMF 1 Before 99 77 78 11.5 After 50 39 78 9.1 0.79 2 Before 11 5 45 0.9 After 0 0 -- 0.0 --- 3 Before 118 47 40 6.0 After 17 4 24 3.6 0.60 4 Before 89 62 70 7.9 After 6 4 67 3.6 0.46 5 Before 430 358 83 39.0 After 5 2 40 2.4 0.06 All Before 747 549 74 65.3 After 78 49 63 18.7 0.29 Despite the impressive CMF of 0.29 for all sites combined, Arnott expresses a need for caution in trusting the result. Sites 3, 4 and 5 have after periods of less than 13 months, which Arnott believes does not yield stable long-term results. Site 1, which has been in operation for over four years, exhibits a 21% reduction in target crashes. Arnott states that this is a more reasonable long-term reduction. Page 63
Traffic Signs Apart from Arnott’s expressed concerns with the results of his study, there is also a high potential for regression-to-the-mean effects to be overestimating the results. The sites were likely selected because of an elevated incidence of crashes. Furthermore, time trends and exposure have not been controlled for at any of the sites. For example, at Site 1 the 22 non-target crashes (99 total crashes minus the 77 target crashes), while not supposedly impacted by the treatment, decreased by 22% from the before to after periods. This is almost identical to the 21% decrease observed in the target crashes group. One cannot comfortably conclude that the traffic activated curve speed warning signs had any significant impact on safety. Zador et al (1987) Zador et el (1987) completed a study to compare driver behaviour at horizontal curves outfitted with chevrons, post-mounted delineators (PMD), and raised pavement markers (RPM). The devices were independently installed at sites that varied systematically in direction, degree of curvature, and steepness of grade. The study used before and after data on lane placement and speed with a comparison group. There were 51 sites located on two-lane rural roads. The treatments were installed in accordance with the requirements of the FHWA MUTCD. All sites, including the comparison sites, had edge line markings. The treatments being evaluated are as follows: • RPMs: 4”x4” amber markers installed on both sides of a double yellow directional dividing line and throughout the curve. The RPMs were visible to both directions of travel and were recessed into the pavement. Spacing was typically 80 feet; along sharper curves where the 80 foot spacing did not result in at least three RPMs being visible at all times, the spacing was decreased to 40 feet. • PMDs: White, round delineators with a 3 inch diameter on metal posts were installed on the outside of the curve. PMDs were visible to drivers from both directions. Spacing was determined such that a motorist could view at least three delineators simultaneously. • Chevrons: 18” x 24” chevron signs mounted on the outside of the curve and visible to drivers in both directions. Spacing was determined such that a motorist could view at least three chevrons simultaneously. Zador et al found that all of the treatments affected driver speeds at night. The differences between the treatment type and the roadway alignment were few, and they Page 64
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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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Chapter 3: Chapter 3: Intersectio I
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Page 79 and 80: Intersection Control The CMFs are 0
Page 81 and 82: Intersection Control also examined
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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