Synthesis of Safety for Traffic Operations - Transports Canada

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Synthesis of Safety for Traffic Operations March 2003 Advance Warning Flashers Gibby et al (1992) Gibby et al (1992) undertook research into the characteristics of approaches to highspeed, isolated, signalized intersections at 40 locations in California. This analysis included the evaluation of advance warning signs and flashers to these signalized intersections. Ten years of crash data were used, sites were representative of the most and least safe intersections of this type in the California state highway system. The results of the Gibby et al analysis are applicable to locations that are rural, have at least one approach with a posted speed limit of 50 mph or greater, and at least one approach is a state highway. The advance warning flasher (AWF) was classified as an advance warning sign (AWS) such as a “signal ahead” sign that is supplemented by at least one 300 mm flashing amber beacon. The difference in mean crash rates at the different locations were analysed, and are shown in Table 3.39. TABLE 3.39: Crash Rates for AWS and AWF in California Treatment Mean Number of Standard Approach Approaches Deviation Crash Rate None 14 0.84 0.48 AWS 85 2.83 3.10 AWF 77 1.13 1.14 Both AWS + AWF 14 1.57 1.17 The results of the analysis indicate that the installation of AWSs and AWFs at isolated, high-speed signalized intersections increase crash rates. This conclusion seems to be counterintuitive and is likely untrustworthy because of the following two (main) shortcomings in the study design: • The study uses a cross-section rather than a before-after study design to examine the differences in mean crash rates. This type of analysis is not as reliable in controlling for confounding influences between intersections. It is likely that other differences between the study intersections have played some role in shaping the different crash rates. • The allocation of sites to the different treatment groups was likely based on safety performance, and has therefore tainted the analysis. Those sites that have not been treated are likely those with the best safety performance. Page 45
Intersection Control Sayed et al (1999) Sayed et al (1999) examined the safety of providing advance warning flashers (AWFs) at signalized intersections in British Columbia. A total of 106 intersections were used in the analysis; 25 of which were equipped with AWFs. The treatment consists of a rectangular warning sign that is equipped with two amber beacons mounted on either side, that operate in alternate flashing mode. The signs are illuminated and erected overhead, but positioned over the shoulder. The location of the sign and the onset of flashing operation are in accordance with generally accepted Canadian practice. The study methodology included the development of SPFs for intersections with and without AWFs to determine the safety impacts of these devices. The use of properly developed SPFs accounts for regression-to-the-mean. The SPFs were developed and used in three different ways to evaluate the safety impacts (i.e., the form of the SPF was constant among all situations but the researchers experimented with model development by varying or holding constant certain parameters between SPFs). The results are shown in Table 3.40. TABLE 3.40: Safety Impacts of AWFs in British Columbia Method CMF Total Injury+Fatal Rear-end 1 0.92 0.91 1.03 2 0.88 0.86 0.97 3 0.82 0.86 0.92 All three methods show remarkably similar results. Methods 2 and 3 did not produce statistically significant results at the 95% level of confidence. Upon further investigation, Sayed et al determined that the safety impacts of the AWFs were associated with the minor street volume at signalized locations. The results of this more in-depth investigation are shown in Figure 3.1. The results indicate that when minor street volumes are relatively low, and main street volumes are high, the AWFs actually degrade safety. A minor street volume of about 13,000 vehicles per day is required before the AWFs provide a safety benefit for all major street volumes. Page 46
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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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Page 24: Chapter 1: Chapter 1: Introductio I
Page 27 and 28: Introduction Before proceeding, a f
Page 29 and 30: Introduction Literature regarding t
Page 31 and 32: Introduction The current science of
Page 33 and 34: Introduction adequate size control
Page 35 and 36: Introduction In concluding this sec
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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 7: Chapter 7: Bicycle Bicyc
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Bicycle Safety TABLE 7.1: Safety Im
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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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