National Academies Press: OpenBook

Guidance for Evaluating the Safety Impacts of Intersection Sight Distance (2018)

Chapter: Chapter 3 - Safety Performance and Intersection Sight Distance

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Suggested Citation:"Chapter 3 - Safety Performance and Intersection Sight Distance." 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 3 - Safety Performance and Intersection Sight Distance." 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 3 - Safety Performance and Intersection Sight Distance." 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 3 - Safety Performance and Intersection Sight Distance." 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 3 - Safety Performance and Intersection Sight Distance." 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 3 - Safety Performance and Intersection Sight Distance." 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 3 - Safety Performance and Intersection Sight Distance." 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 20
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Suggested Citation:"Chapter 3 - Safety Performance and Intersection Sight Distance." 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 21
Page 22
Suggested Citation:"Chapter 3 - Safety Performance and Intersection Sight Distance." 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 22
Page 23
Suggested Citation:"Chapter 3 - Safety Performance and Intersection Sight Distance." 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 23

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14 Safety Performance and Intersection Sight Distance The purpose of this chapter is to provide practitioners information on the anticipated impact on crashes at an intersection from changing the sight distance (e.g., lengthening the distance at an intersection from 400 ft to 750 ft by removing a sight distance obstruction). The results can be applied to both existing and design-stage intersections. Determining the CMF The following pages contain a series of charts depicting CMFs for target crashes and target fatal and injury crashes. Charts A-1 to A-6 present CMFunctions for target crashes. Charts B-1 to B-6 present CMFunctions for target fatal and injury crashes. The charts are based on the CMFunctions developed from the research and provided for reference in Chapter 6. Detailed instructions are provided in the Practitioner Tips: Detailed Instructions for Using CMFs text- box to help practitioners use these charts in practice, with the key steps identified on the sample chart (Figure 6). For the purposes of these charts, there are several important notes. First, the charts are sepa- rated by major-road posted speed from 35 mph to 60 mph. Each chart refers to a specific posted speed. The minimum ISD for each posted speed corresponds to the design ISD based on left turns for an equivalent design speed minus 250 ft (as data were limited below this distance). The maximum ISD value is one-quarter mile (1,320 ft), which is the base condition assumed in the charts. However, direct comparisons may be made between two alternative CMFs on a chart, as shown in the examples. Additionally, outcome crashes are all target crashes (i.e., a crash involv- ing a vehicle on the major road and a vehicle on the minor road). Therefore, it is essential that the approach and direction of travel be known. For example, left-looking sight distance on one approach is only applicable for crashes involving a vehicle from that minor approach and a vehicle from the major road entering from the left. For a four-leg intersection, the vehicle entering from the minor road needs to be applied to the correct minor-road approach leg. More detailed scenario examples—as well as an in-depth look at how to plot existing and proposed ISD and how to calculate the appropriate CMF—are provided in Chapter 4. C h a p t e r 3

Safety performance and Intersection Sight Distance 15 Practitioner Tips: Detailed Instructions for Using CMFs The following information provides step-by-step guidance on how to use the charts to determine the crash impact from changing the existing ISD to a new proposed ISD. A sample graph (Figure 6) is also provided to help illustrate the following steps. Steps 1 through 6 correspond to a red number in Figure 6. Step 1. Identify the minor-road approach sight distance of interest (left-looking or right-looking) for which a new ISD is proposed. Measure the sight distance along the major road for that direction using the method in Chapter 2. This is the existing ISD for the approach and direction. Step 2. Identify the type of crash of interest for the analysis: target or target fatal or injury crash. • Target crashes are defined as a crash involving a vehicle from the major road and a vehicle entering from the minor road. • Target fatal and injury crashes are a subset of target crashes that involve one or more injuries or fatalities. Step 3. Identify the appropriate chart type based on the type of crash and major- road posted speed. Step 4. Identify the curve that corresponds to the appropriate major-road AADT range. • Three ranges of major-road AADT are used for target crashes. • Two ranges of major-road AADT are used for target fatal and injury crashes. Step 5. Using the selected curve, plot the existing and the proposed ISD. If any ISD exceeds 1,320 ft, use 1,320 ft in the chart since this is the maximum. Step 6. Calculate the CMF for changing the site distance from the existing to the proposed by dividing the CMF from the chart for the proposed ISD by the CMF from the chart for the existing ISD. Step 7. Apply the resulting CMF to the crashes of interest associated with the corresponding direction. These steps are undertaken for each approach and direction of interest at the intersection. When analyzing the whole intersection, carry out this process for each minor approach and direction (left-looking or right-looking) and combine the results. For a four-leg intersection, these steps would be undertaken four times.

16 Guidance for evaluating the Safety Impacts of Intersection Sight Distance Figure 6. Sample chart. Chart A-1. CMFs for target crashes when posted speed equals 35 mph.

Safety performance and Intersection Sight Distance 17 Chart A-2. CMFs for target crashes when posted speed equals 40 mph. Chart A-3. CMFs for target crashes when posted speed equals 45 mph.

18 Guidance for evaluating the Safety Impacts of Intersection Sight Distance Chart A-4. CMFs for target crashes when posted speed equals 50 mph. Chart A-5. CMFs for target crashes when posted speed equals 55 mph.

Safety performance and Intersection Sight Distance 19 Chart A-6. CMFs for target crashes when posted speed equals 60 mph. Chart B-1. CMFs for target fatal and injury crashes when posted speed equals 35 mph.

20 Guidance for evaluating the Safety Impacts of Intersection Sight Distance Chart B-2. CMFs for target fatal and injury crashes when posted speed equals 40 mph. Chart B-3. CMFs for target fatal and injury crashes when posted speed equals 45 mph.

Safety performance and Intersection Sight Distance 21 Chart B-4. CMFs for target fatal and injury crashes when posted speed equals 50 mph. Chart B-5. CMFs for target fatal and injury crashes when posted speed equals 55 mph.

22 Guidance for evaluating the Safety Impacts of Intersection Sight Distance Analyzing More Than One Approach The crash modification factor for intersection-level target crashes can be calculated using the following equation: ∑ ∑= × CMF CMF T T IT T i i i where: CMFIT = crash modification factor for intersection-level target crashes, CMFTi = crash modification factor for target crashes for approach direction i, and Ti = target crashes for approach direction i. If the intersection has three legs, there will be two approach directions. If the intersection has four legs, there will be four approach directions. If only one ISD is being changed, only one CMFTi need be calculated. A value of 1.0 may be used for the other approach directions. Exam- ple 4 presents the calculation of an intersection-level total crashes CMF from the target crashes application. Note that if no crash data exist for the intersection, the CMF for intersection-level target crashes can be determined by averaging the CMFs for all approach directions. To determine the CMF for total intersection crashes, use the following equation: 1.0 1.0CMF CMF PIT T( )= − × + Chart B-6. CMFs for target fatal and injury crashes when posted speed equals 60 mph.

Safety performance and Intersection Sight Distance 23 where: CMF = crash modification factor total intersection crashes, and PT = proportion of total intersection crashes that are target crashes. This equation assumes that only target crashes will be affected by the change to ISD and requires the CMF for intersection-level target crashes and the proportion of total intersection crashes that are target crashes. An average proportion for the intersection type may be used if the proportion is unknown for the analysis intersection.

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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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