Guidance for Evaluating the Safety Impacts of Intersection Sight Distance (2018) / Chapter Skim
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24 Examples The examples presented in this chapter demonstrate how practitioners can apply the charts presented in Chapter 3. Example 1: One Direction ISD Upgrade A three-leg intersection with stop control on the minor-road approach has a major-road AADT of 7,000 and a major-road posted speed of 55 mph.
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examples 25 Figure 7. Example 1: target crashes.
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26 Guidance for evaluating the Safety Impacts of Intersection Sight Distance for the proposed ISD of 600 ft is 1.30. The CMF for the existing ISD of 250 ft is 2.54.
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examples 27 Target Fatal and Injury Crashes The same steps are undertaken if the practitioner is interested in target crashes that resulted in a fatality or injury. Instead of Chart A-2 (which was for all target crashes)
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28 Guidance for evaluating the Safety Impacts of Intersection Sight Distance Example 3: Calculation of Total Crashes CMF A four-leg intersection with stop control on the minor road has a major-road AADT of 17,500 and a posted speed of 60 mph. In a 3-year period, the southbound left-looking ISD from the minor road has experienced five target crashes, the right-looking ISD has experienced seven target crashes, and the intersection has experienced 16 target crashes.
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examples 29 Example 2 illustrated how to combine the CMFs to determine an intersection-level CMF for the target crashes. The same approach is taken in this example.
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30 Guidance for evaluating the Safety Impacts of Intersection Sight Distance The following conditions apply: • The posted speed is 50 mph, • This site uses the LowAADT for the major road (i.e., LowAADTmaj = 1.0) , • The proposed ISD is 465 ft for this analysis, • The existing ISD is 555 ft based on the design ISD, and • The base ISD is 1,320 ft.
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examples 31 This results in the following CMF for the proposed ISD condition: = − × + × + − × − × + × + − × = exp 0.009 50 6.335 50 465 155.504 1 465 exp 0.009 50 6.335 50 1,320 155.504 1 1,320 1.251CMFTFI The CMF for the existing ISD condition is: = − × + × + − × − × + × + − × = exp 0.009 50 6.335 50 555 155.504 1 555 exp 0.009 50 6.335 50 1,320 155.504 1 1,320 1.182CMFTFI The target fatal and injury crashes CMF is calculated as: = = 1.25 1.18 1.06CMFT The target fatal and injury crashes CMF for having an ISD of less than the design ISD is 1.06. This factor applies to fatal and injury multi-vehicle crashes involving vehicles from the minor road and major-road vehicles approaching from the right.
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