National Academies Press: OpenBook

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

Chapter: Chapter 6 - Base Equations for Reference

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Suggested Citation:"Chapter 6 - Base Equations for Reference." 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 34
Page 35
Suggested Citation:"Chapter 6 - Base Equations for Reference." 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 35

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34 Base Equations for Reference CMFunctions Charts A-1 to A-6 present CMFunctions based on the following equation for target crashes: = − × + × + − × + − ×       − × + × + − × + − ×       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 where: CMFTi = target crash CMF for condition of interest i; PSL = posted speed (in mph); LowAADTmaj = 1 if major-road AADT ≤ 5,000; otherwise 0; MidAADTmaj = 1 if 5,000 < major-road AADT ≤ 15,000; otherwise 0; ISDi = proposed or existing available intersection sight distance for the condition of interest i (in feet); and ISDbase = base intersection sight distance for an approach direction (in feet); for practi- cal applications, this value is assumed to be 1,320 ft. Charts B-1 to B-6 present CMFunctions based on the following equation 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 = − × + × + − ×  − × + × + − ×  where: CMFTFIi = target fatal and injury crash CMF for condition of interest i; and LowMidAADTmaj = 1 if major-road AADT ≤ 15,000; otherwise 0. Caution should be used for posted speeds outside of the 35 to 60 mph range used to develop the charts. Additionally, ISDs used in the CMFunctions should only consider the ranges provided C h a P t e r 6

Base equations for reference 35 in the charts. The minimum ISD for each posted speed corresponds to the design ISD based on left turns for an equivalent design speed minus 250 ft. The maximum ISD value is one-quarter mile (1,320 ft), which is the base condition assumed in the provided charts. Unknown AADT If AADT and posted speed are known, the charts provide a more informed estimate of safety impacts. Reduced versions of the CMFunctions may be applied for situations where major AADT and posted speed are unknown. The equations can be assessed for target crashes and target fatal and injury crashes. Target crashes: The impact of changing ISD can be assessed for target crashes using the fol- lowing equation: = × −       203.368 1 1 CMF eT ISD ISDproposed existing For example, the CMF for increasing ISD from an existing ISD condition of 400 ft to a pro- posed ISD condition of 750 ft is calculated as: = × −       203.368 1 750 1 400CMF eT This results in a CMF of 0.79, or a 21% decrease in target crashes. Target fatal and injury crashes: The impact of changing ISD can also be assessed for target fatal and injury crashes using the following equation: = × −      195.791 1 1 CMF eTFI ISD ISDproposed existing The CMF for increasing ISD from an existing ISD condition of 400 ft to a proposed ISD condi- tion of 750 ft is calculated as: = × −      195.791 1 750 1 400CMF eTFI This results in a CMF of 0.80, or a 20% decrease in fatal and injury target crashes.

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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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