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Page 24
Suggested Citation:"Chapter 4 - Examples." National Academies of Sciences, Engineering, and Medicine. 2018. Guidance for Evaluating the Safety Impacts of Intersection Sight Distance. Washington, DC: The National Academies Press. doi: 10.17226/25081.
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Suggested Citation:"Chapter 4 - Examples." National Academies of Sciences, Engineering, and Medicine. 2018. Guidance for Evaluating the Safety Impacts of Intersection Sight Distance. Washington, DC: The National Academies Press. doi: 10.17226/25081.
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Suggested Citation:"Chapter 4 - Examples." National Academies of Sciences, Engineering, and Medicine. 2018. Guidance for Evaluating the Safety Impacts of Intersection Sight Distance. Washington, DC: The National Academies Press. doi: 10.17226/25081.
×
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Suggested Citation:"Chapter 4 - Examples." National Academies of Sciences, Engineering, and Medicine. 2018. Guidance for Evaluating the Safety Impacts of Intersection Sight Distance. Washington, DC: The National Academies Press. doi: 10.17226/25081.
×
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Suggested Citation:"Chapter 4 - Examples." National Academies of Sciences, Engineering, and Medicine. 2018. Guidance for Evaluating the Safety Impacts of Intersection Sight Distance. Washington, DC: The National Academies Press. doi: 10.17226/25081.
×
Page 28
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Suggested Citation:"Chapter 4 - Examples." National Academies of Sciences, Engineering, and Medicine. 2018. Guidance for Evaluating the Safety Impacts of Intersection Sight Distance. Washington, DC: The National Academies Press. doi: 10.17226/25081.
×
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Suggested Citation:"Chapter 4 - Examples." National Academies of Sciences, Engineering, and Medicine. 2018. Guidance for Evaluating the Safety Impacts of Intersection Sight Distance. Washington, DC: The National Academies Press. doi: 10.17226/25081.
×
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Suggested Citation:"Chapter 4 - Examples." National Academies of Sciences, Engineering, and Medicine. 2018. Guidance for Evaluating the Safety Impacts of Intersection Sight Distance. Washington, DC: The National Academies Press. doi: 10.17226/25081.
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Page 31

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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. A practitioner is interested in esti- mating the change in average target crash frequency when increasing left-looking ISD from an existing condition of 400 ft to a proposed condition of 750 ft. Using Chart A-5 for 55 mph (shown here as Figure 7 with some annotation), the CMF for the proposed ISD of 750 ft is 1.13. The CMF for the existing ISD of 400 ft is 1.46. Therefore, the target crashes CMF for improving ISD is 1.13/1.46, or 0.77. This factor applies to multi-vehicle crashes involving vehicles from the minor road and vehicles approaching from the left. The increase in sight distance from 400 ft to 750 ft in this example would reduce target crashes on the left-looking direction of the approach by 23%. Example 2: Upgrading Both Left and Right Departure Sight Triangles from Different Values A four-leg intersection with stop control on the minor road has a major-road AADT of 20,000 and posted speed of 40 mph. The intersection was selected for potential improvement due to a history of gap-acceptance–related crashes. Figure 8 presents a crash diagram of target crashes and their severities for the three most recent years of crash data. The crash diagram shows that the northbound minor-road approach has four crashes associated with the left- looking ISD and five crashes associated with the right-looking ISD. The northbound left-looking ISD is being considered for improvement from an existing ISD condition of 250 ft to a pro- posed ISD condition of 600 ft. Additionally, the right-looking ISD is being considered for improvement from an existing ISD condition of 300 ft to a proposed ISD condition of 600 ft. A practitioner wants to determine the intersection-level target crash CMF and intersection-level target fatal and injury crash CMF. Target Crashes Each direction for the approach is analyzed separately, starting with the left-looking ISD. Using Chart A-2 (shown here as Figure 9 with some annotation), the left-looking direction CMF C h a p t e r 4

examples 25 Figure 7. Example 1: target crashes. Figure 8. Example 2: crash diagram.

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. Therefore, the CMF for target crashes for improving the left-looking ISD on the approach is 1.30/2.54, or 0.51. This CMF applies only to the four crashes in Figure 8 involving vehicles on the minor road and vehicles on the major road approaching from the left. The right-looking direction CMF is calculated from the same chart. The CMF for the pro- posed ISD of 600 ft is 1.30. The CMF for the existing ISD of 300 ft is 2.10. Therefore, the CMF for target crashes for improving the right-looking ISD on the approach is 1.30/2.10, or 0.62. This CMF applies only to the five crashes in Figure 8 involving vehicles on the minor road and vehicles on the major road approaching from the right. Combining CMFs Once the practitioner has the left-looking CMF and the right-looking CMF for the approach, he or she may want to combine those to obtain a target crash CMF for the entire intersection. For this example, two of the four approach directions were modified. The following equation computes the intersection-level CMF for target crashes based on the individual approach direc- tion CMFs and target crash history. 0.51 4 0.62 5 1 1 1 0 4 5 1 0 CMF crashes crashes crash crashes IT ( ) ( ) ( ) ( ) ( )= ∗ + ∗ + ∗ + ∗ + + + The resulting intersection-level CMF for target crashes is 0.61. Figure 9. Example 2: target crashes.

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), Chart B-2 is used because it is applicable for target fatal and injury crashes with a posted speed limit of 40 mph. The chart is annotated and included here as Figure 10. Again, each approach is analyzed sepa- rately. The left-looking direction CMF for the proposed ISD of 600 ft is 1.26. The CMF for the existing ISD of 250 ft is 2.27. Therefore, the CMF for target fatal and injury crashes for improv- ing the left-looking ISD on the approach is 1.26/2.27, or 0.56. This CMF applies only to the two fatal and injury crashes involving vehicles on the minor road and vehicles on the major road approaching from the left. The right-looking direction CMF is calculated from the same chart. The CMF for the pro- posed ISD of 600 ft is 1.26. The CMF for the existing ISD of 300 ft is 1.92. Therefore, the CMF for improving the right-looking ISD on the approach is 1.26/1.92, or 0.66. This CMF applies only to the three fatal and injury crashes involving vehicles on the minor road and vehicles on the major road approaching from the right. Using the same equations and methods to combine the CMFs to get an intersection-level CMF for target fatal and injury crashes, the resulting calculation is: 0.56 2 0.66 3 1 0 1 0 2 3 0 0 CMF crashes crashes crashes IT ( ) ( ) ( ) ( ) ( )= ∗ + ∗ + ∗ + ∗ + + + The resulting intersection-level CMF for target fatal and injury crashes is 0.62. Figure 10. Example 2: target fatal and injury crashes.

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. The remaining four crashes involved the left-looking northbound approach. The right-looking northbound approach has not experienced any target crashes. The southbound minor-road approach is being considered for improvements that would increase the existing left-looking ISD condi- tion from 525 ft to a proposed ISD condition of 1,320 ft. Additionally, the right-looking ISD is being considered for improvements that would increase the existing ISD condition from 610 ft to a proposed ISD condition of 1,320 ft. A practitioner is interested in calculating the CMFs for target crashes for each of the ISD improvements as well as an estimate of the intersection- level total crash CMF for these improvements based on data indicating that 55% of the total intersection crashes are target crashes. Using Figure 11, the left-looking direction CMF for the proposed ISD of 1,320 ft is 1.00. The CMF for the existing ISD of 525 ft is 1.64. Therefore, the CMF for improving left-looking ISD is 1.00/1.64, or 0.61. This CMF applies only to multi-vehicle crashes involving vehicles on the minor road and vehicles on the major road approaching from the left. The right-looking direc- tion CMF for the proposed ISD of 1,320 ft is 1.00. The CMF for the existing ISD of 610 ft is 1.46. Therefore, the CMF for improving the right-looking ISD is 1.00/1.46, or 0.68. This CMF applies only to multi-vehicle crashes involving vehicles on the minor road and vehicles on the major road approaching from the right. Figure 11. Example 3: target crashes.

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. The CMF for intersection-level target crashes is calculated using the following equation: ( ) ( ) ( ) ( ) = × + × + × + × + + + 0.61 5 0.68 7 1 4 1 0 5 7 4 0 CMFIT Therefore, the CMF for intersection-level target crashes is 0.74. The practitioner is also interested in calculating an intersection-level CMF for all crashes, not just target crashes. This is accomplished by applying the CMF for intersection-level target crashes to the following equation. ( )= − × +1.0 1.0CMF CMF PIT T The proportion of target crashes to total crashes is given as 0.55. Substituting these values yields the following equation. ( )= − × +0.74 1.0 0.55 1.0CMF The CMF for total crashes is 0.86. This indicates that the upgrades to two quadrants of ISD will result in a 26% reduction in target crashes and a 14% reduction in total crashes. Example 4: Design Deviation Using Equations The charts presented in Chapter 3 are based on the CMFunctions that were developed from the research. The equations are included in Chapter 6 for reference. All the CMFs developed from this research can be extracted from the charts without the use of the equations. However, for the practitioners’ reference, this example illustrates how the equations are used. A three-leg intersection with stop control on the minor road has a major-road AADT of 1,200 and a posted speed of 50 mph. Due to the presence of a roadside embankment, a practitioner is estimating the potential safety impact of having a right-looking available ISD of 465 ft in com- parison to a design ISD of 555 ft. The analysis focuses on directly applying the target crashes and target fatal and injury crashes CMFunctions. Target Crashes Use the following to compute the CMF for target crashes of interest, either for proposed or existing conditions: exp 0.021 7.194 243.009 177.826 exp 0.021 7.194 243.009 177.826 CMF PSL PSL ISD LowAADT ISD MidAADT ISD PSL PSL ISD LowAADT ISD MidAADT ISD T i maj i maj i base maj base maj base i = − × + × + − × + − ×       − × + × + − × + − ×      

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. This results in the following CMF for the proposed ISD condition: exp 0.021 50 7.194 50 465 243.009 1 465 177.826 0 465 exp 0.021 50 7.194 50 1,320 243.009 1 1,320 177.826 0 1,320 1.181CMFT = − × + × + − × + − ×    − × + × + − × + − ×  = The CMF for the existing ISD condition is: exp 0.021 50 7.194 50 555 243.009 1 555 177.826 0 555 exp 0.021 50 7.194 50 1,320 243.009 1 1,320 177.826 0 1,320 1.132CMFT = − × + × + − × + − ×    − × + × + − × + − ×  = The target fatal and injury crashes CMF is calculated as: = = 1.18 1.13 1.04CMFT The target fatal and injury crashes CMF for having an ISD of less than the design ISD is 1.04. This factor applies to multi-vehicle crashes involving vehicles from the minor road and major- road vehicles approaching from the right. Note that the CMF can also be computed directly by using the existing ISD as the base ISD as follows: = − × + × + − × + − ×    − × + × + − × + − ×    = exp 0.021 50 7.194 50 465 243.009 1 465 177.826 0 465 exp 0.021 50 7.194 50 555 243.009 1 555 177.826 0 555 1.04CMFT Target Fatal and Injury Crashes Use the following equation to compute the CMF for target fatal and injury crashes: = − × + × + − ×  − × + × + − ×  exp 0.009 6.335 155.504 exp 0.009 6.335 155.504 CMF PSL PSL ISD LowMidAADT ISD PSL PSL ISD LowMidAADT ISD TFI i maj i base maj base i The following conditions apply: • The posted speed is 50 mph, • This site uses the LowMidAADT for the major road (i.e., LowMidAADTmaj = 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.

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. Note that the CMF can also be com- puted directly by using the existing ISD as the base ISD as follows: = − × + × + − ×    − × + × + − ×    = exp 0.009 50 6.335 50 465 155.504 1 465 exp 0.009 50 6.335 50 555 155.504 1 555 1.06CMFTFI

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TRB's National Cooperative Highway Research Program (NCHRP) Research Report 875: Guidance for Evaluating the Safety Impacts of Intersection Sight Distance is a resource for practitioners involved in the planning, design, operations, and traffic safety management of stop-controlled intersections. It provides information on how to estimate the effect of intersection sight distance (ISD) on crash frequency at intersections and describes data collection methods and analysis steps for making safety-informed decisions about ISD. The guidance also provides basic information on the importance of ISD that can be shared with decision makers and other stakeholders.

Accompanying the report, NCHRP Web-Only Document 228: Safety Impacts of Intersection Sight Distance documents the methodology and presents the results from the underlying research on estimating the safety effects of ISD at stop-controlled intersections. To establish the relationship between ISD and safety at stop-controlled intersections, crash, traffic, and geometric data were collected for 832 intersection approaches with minor-road stop control in North Carolina, Ohio, and Washington. A PowerPoint presentation that describes the project also accompanies the report and web-only document.

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