Development of Left-Turn Lane Warrants for Unsignalized Intersections (2013) / Chapter Skim
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Pages 83-132
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81 CHAPTER 5 DELAY, CRASH, AND CONSTRUCTION COST STUDIES Left-turn lanes can provide benefits in safety as well as operations. Left-turn lanes can reduce the potential for collisions by providing safer left-turn operations.
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82 Where: B/C = benefit-cost ratio; DR(VTR, VLT, Hr) = delay reduction (veh-hr/year)
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83 In the second scenario, the baseline condition is that no delay is present at the location because there is no left-turn demand. When a new development is proposed, the added delay to the system is determined from simulation.
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84 Figure 23. Simulation-estimated delay reduction when adding a left-turn lane at an existing site.
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85 Figure 24. Simulation-estimated added delay for a new development (no left-turn lane scenario)
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86 Table 45. Regression coefficients to predict the delay determined from simulation.
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87 The following is an illustration of the use of the equation for delay reduction when there are 600 veh/hr/ln on the major roadway and 60 left-turning vehicles during the peak hour: DR2 ln, 40 mph = −2.30383 + 0.00395 MajorPHV + 0.01289 LTLPHV (23)
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88 Table 47. Factors used to convert sec/veh delay to hr/intersection delay for a year.
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89 variable estimated is the predicted average crash frequency for a roadway segment or intersection under base conditions, and the independent variables are the AADTs of the roadway segment or intersection legs (and, for roadway segments, the length of the roadway segment)
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90 Table 50. Safety performance functions for urban and suburban arterials for total crashes.
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91 Table 51. Definitions for variables in Table 50.
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92 Table 52. Minimum and maximum AADT for Highway Safety Manual equations.
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93 Figure 25. Illustration of predicted crash frequency using Highway Safety Manual equations.
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94 studies. Figure 26 illustrates the predicted average total crashes and actual total crashes (average based on 5 years of data)
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95 This section describes the methodology researchers used to develop an estimate of the value of a statistical life (VSL) in 2009 dollars by crash severity.
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96 Table 53. For example, the low disabling injury (A)
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97 Table 56. Cost difference between comprehensive societal cost estimates and human capital cost estimates (2008 dollars)
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98 Table 58. 2009 ECI-adjusted cost difference between comprehensive societal cost estimates and human capital cost estimates (2009 dollars)
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99 Table 60. Default distribution of crash severity level at rural two-lane two-way intersections from the Highway Safety Manual (77)
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100 Table 62. Injuries or deaths per crash for rural intersections.
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101 Table 63. Typical crash cost calculations for three-leg rural intersections.
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102 Table 64. Typical crash cost calculations for four-leg intersections.
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103 Table 65. Typical crash cost calculations for urban and suburban intersections.
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104 Table 67. Typical crash cost by number of legs and rural or urban based on Highway Safety Manual crash costs.
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105 Table 68. Estimated construction cost from literature.
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106 EXAMPLES OF CALCULATIONS FOR ADDING LEFT-TURN LANE AT EXISTING SITE Adding Left-Turn Lane at Existing Site on Rural Two-Lane Highway Annual Delay Savings The following illustrates the calculations to determine if a left-turn lane should be considered at a rural two-lane highway site. Assume that the conditions at the site include the following: • Two lanes, • Rural location, • Three legs, • 50-mph posted speed limit, • 450 veh/hr/ln in the peak hour (10,000 ADT on the major road)
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107 Table 70. Calculation of annual delay savings for rural two-lane highway example.
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108 The B/C ratio when using the mid-range societal cost is: ൌ ଵଷ.ହଽ ൈ ሺ$ସ,ଶଵସ ା $ଶଽସ,ହଽሻ $ଶହ, ൌ 16.2 (53) The B/C ratio of 16.2 indicates that the installation of a left-turn lane should be warranted for these conditions.
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109 Table 71. Calculation of annual delay savings for rural four-lane highway example.
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110 The B/C ratio when using the mid-range societal cost is: ൌ ଵଷ.ହଽ ൈ ሺ$ଵ,ହଵସ ା $ଷ,଼ଽହሻ $ଶହ, ൌ 20.6 (57) The B/C ratio of 20.6 indicates that the installation of a left-turn lane should be warranted for these conditions.
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111 Annual Crash Savings The determination of the savings attributed to crashes begins with calculating the predicted number of crashes for the intersection. For urban or suburban intersections, the predicted number of crashes consists of the predicted number of multiple-vehicle crashes, predicted number of single-vehicle crashes, predicted number of pedestrian-vehicle crashes, and predicted number of bicycle-vehicle crashes both before and after a left-turn lane is installed.
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112 The costs per crash at three-leg urban and suburban intersections are: $167,000 (mid-range) , $92,000 (low range)
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113 Figure 27. Plot of Green Book rural two-lane highway left-turn warrant values.
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114 DEVELOP PRELIMINARY WARRANTS FOR LEFT-TURN LANE The benefit-cost methodology presented above was applied to a range of major-road ADTs (1000 to 15,000) and minor-road ADTs (200 to 3000)
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115 The figures for rural two-lane highways or urban and suburban arterials also include a representation of the current Green Book warrants. Two lanes, three legs Two lanes, four legs Figure 29.
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116 Four lanes, three legs Four lanes, four legs Figure 30. Range of left-turn lane warrants based on crash costs (low, mid-, and high range)
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117 Three legs Four legs Figure 31. Range of left-turn lane warrants based on crash costs (low, mid-, and high range)
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118 Two lanes, three legs Two lanes, four legs Figure 32. Range of left-turn lane warrants based on construction costs (minimum, moderate, and maximum)
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119 Four lanes, three legs Four lanes, four legs Figure 33. Range of left-turn lane warrants based on construction costs (minimum, moderate, and maximum)
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120 Three legs Four legs Figure 34. Range of left-turn lane warrants based on construction costs (minimum, moderate, and maximum)
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121 The lines shown in the plots represent the minimum volumes for which a left-turn lane would be recommended based on anticipated delay reduction and crash reduction from adding a left-turn lane to an existing site. In each plot, the point corresponding to the peak-hour major-road volume and the peak-hour left-turn volume should be located.
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122 Suggested left-turn warrants for urban and suburban arterials Suggested left-turn warrants for rural highways Figure 35. Suggested left-turn warrants based on results from benefit-cost evaluation when using B/C of 1.0 and mid-range crash cost and moderate construction cost.
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123 Suggested left-turn warrants for urban and suburban arterials Suggested left-turn warrants for rural highways Figure 36. Suggested left-turn warrants based on results from benefit-cost evaluation when using B/C of 2.0 and mid-range crash cost and moderate construction cost.
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124 Table 73. Range of left-turn lane warrants based on results from benefit-cost evaluations using crash costs developed based on FHWA economic value of a statistical life.
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125 Since the Highway Safety Manual equations are used, some may argue that the Highway Safety Manual costs should also be used. The warrants resulting from assuming the Highway Safety Manual crash costs (adjusted to 2009 dollars)
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126 Suggested left-turn warrants for urban and suburban arterials Suggested left-turn warrants for rural highways Figure 37. Suggested left-turn warrants based on results from benefit-cost evaluation when using B/C of 1.0 and HSM crash costs (2009 dollars)
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127 other words, what is the tradeoff between the benefits that may be achieved and the costs of providing a left-turn lane? For the installation of a left-turn lane at a new development, there are additional questions to answer.
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128 question is, if the left-turn inbound movement is allowed, should a left-turn lane be provided, and who should pay for its construction. Example of New Development Additional Annual Delay The findings from this study can also be used to estimate the additional costs to a site if a new development causes left-turn volumes to increase.
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129 Table 75. Calculation of annual delay for new development example.
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130 Present Worth of Annual Costs The delay and crash costs above represent the expected value per year. Assuming a 20-year design life and a 4 percent return, those annual costs can be converted to a present worth as follows: ܲݎ݁ݏ݁݊ݐ ܹݎݐ݄ ݂ ܥݏݐݏ ൌ 13.59 ൈ ሺ$879 $644,324ሻ ൌ $8,768,522 (71)
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