Synthesis of Safety for Traffic Operations - Transports Canada

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Synthesis of Safety for Traffic Operations March 2003 APPENDIX D – HOW TO USE SAFETY PERFORMANCE FUNCTIONS The current direction in safety research and evaluation is to make use of crash prediction equations, also known as safety performance functions (SPFs), to estimate the long-term crash frequency of a facility. SPFs may take many forms, the most basic uses traffic volumes as an independent variable, and crash frequency as the dependent variable. Equations D1 and D2 below, are examples of basic SPFs for road segments and intersections, respectively. N = a ADT b N = a ADT m b ADT s c [D1] [D2] where: N = Crashes/year/km (Equation D1); crashes/year (Equation D2) ADT = Average daily traffic ADT m = Average daily traffic for the main road ADT s = Average daily traffic for the side road a, b, c = constants derived from regression The SPFs are developed through an examination of crash and volume records for a category of roads or intersections. For instance, three-leg, all-way stop controlled intersections in an urban setting may be a category of intersection for which an SPF is developed. Using appropriate regression techniques, data that is available from all intersections in this category can be used to determine the expected crash frequency for this group of facilities. Combining SPF results with Crash Records If the crash record of a site or facility is unavailable, then the output from the SPF is the best estimate of the long-term crash count. For instance, if a new signalized intersection is proposed where one does not exist now, then the SPF for signalized intersections and traffic volume projections can be used to estimate the crash count. However, if the crash record is available, then it must be considered in determining the expected long-term crash count of the facility. The expected frequency as determined by the SPF is an estimate of the safety of the facility, but in order to provide a better estimate of safety for an existing site, the results of the SPF calculation must be tempered with the actual crash record of the site. Consider the following, the SPF for two-lane rural, arterial roads indicates that the expected crash frequency for Arterial “X” is 1.2 crashes/km/yr; the actual crash record of Arterial “X” over the past three years shows 2.6 crashes/km/yr. Which is the correct Page 203
How to Use Safety Performance Functions estimate of the long-term safety? Neither – in order to provide the best estimate of safety the two pieces of information have to be combined. The SPF produces the expected average crash frequency for facilities of the type specified; the crash record of the specific facility is an indication of the safety for the location. By combining these two pieces of information, we arrive at the best estimate of the safety of the facility. In order to join the two numbers, consideration must be given to the: • Reliability of the SPF (how well does it predict crash frequency?); and • Number of years of crash data available for the site. Using both of these factors one can determine the weight to placed on the actual crash record and the weight to be placed on the predicted or expected crash count. In the regression calibration process the mean and the variance of the regression estimate can be used to determine the overdispersion parameter “k”. Further explanation of “k” and statistical modelling surrounding it are left to others [Hauer, 1997]. Nonetheless, “k” is a measure of the reliability of the SPF, and by knowing it and the crash record of the site we can estimate the safety by combining the results of the SPF and the crash record as follows: EC = w OP +(1-w) N/n w = k / { k + (n OP)} [D3] [D4] where: EC = Expected number of crashes/year OP = output from SPF k = statistical measure of overdispersion associated with the SPF N = Actual number of crashes n = number of years of crash data As the number of years of crash data from the site under analysis increases, the weight placed on the measured crash count from motor vehicle crash reports also increases. Example Here is an example of how to use this methodology. An unsignalized intersection has the following characteristics: SPF = 0.0044 ADT m 0.64 ADT s 0.17 [D5] Page 204
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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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Intersection Control TABLE 3.3: Cra
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Intersection Control TABLE 3.5: CMF
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Intersection Control Vancouver Regi
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Intersection Control V m = the sum
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Intersection Control where: X1 = 0
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Intersection Control Region of Durh
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Intersection Control TABLE 3.17: CM
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Intersection Control The predictor
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Intersection Control program includ
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Intersection Control TABLE 3.26: Me
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Intersection Control TABLE 3.29: St
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Intersection Control TABLE 3.32: Ni
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Intersection Control It is noted th
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Intersection Control Sayed et al (1
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Intersection Control Signal Clearan
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Intersection Control The CMFs are 0
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Intersection Control also examined
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Intersection Control Shebeeb (1995)
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Intersection Control approaches at
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Intersection Control TABLE 3.56: Sa
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Chapter 5: Chapter 5: Pavement Pave
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Pavement Markings The study methodo
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Pavement Markings Hall (1987) Hall
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Pavement Markings TABLE 5.8: Safety
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Pavement Markings THIS PAGE IS INTE
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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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Page 208 and 209: Synthesis of Safety for Traffic Ope
Page 220: References References
Page 223 and 224: References Bloch SA (1998) “Compa
Page 225 and 226: References Hamilton Associates (199
Page 227 and 228: References Lee JT, Maleck TL, and T
Page 229 and 230: References Polanis SF (1998) “Do
Page 231 and 232: References Vancouver, City of (1999
Page 233 and 234: References THIS PAGE IS INTENTIONAL
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Page 245 and 246: Critical Reviews Worksheet for Eval
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Synthesis of Safety for Traffic Operations - Transports Canada

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