Manual gives realistic estimate of how changes to intersections will affect crash frequency
Decreasing the frequency of crashes at an intersection is a primary goal when considering changes to an intersection. Changes may include converting an intersection’s control method to a stop-controlled (two-way or all-way), signalized or roundabout configuration. Other changes may include adding left and or right-turn lanes, prohibiting left-turns, and adding overhead lighting or bicycle lanes.
The Highway Safety Manual, published in 2010, provides fairly recent methodology for quantifying expected changes in crash frequency at intersections. The quantification of this effect is referred to as a crash modification factor (CMF). The CMF is used to identify the expected number of crashes per year after implementation of the change. For example the CMF for converting a two-way stop-controlled intersection to a roundabout is 0.61, which indicates the number of crashes per year is expected to be reduced by approximately 39% as a result of the change.
At least 5 years of crash data is typically used to identify trends that may be correctable by implementing a change to an intersection. Examples include:
- Example 1: There is an intersection where most of the crashes are occurring at night. An improvement to correct this trend may be to install a streetlight near the intersection.
- Example 2: There is a stop-controlled intersection where broadside crashes are regularly occurring and result in injuries. An improvement to correct this trend may be to convert the intersection to a roundabout. Roundabouts generally reduce the frequency and severity of crashes over stop-controlled intersections due to fewer conflict points and lower vehicular speeds transiting the intersection.
Implementing changes to an intersection in order to reduce its frequency of crashes is by no means new. Prior to the release of the Highway Safety Manual engineers and planners drew upon personal experience, which could easily be dismissed as subjective or anecdotal. The Highway Safety Manual is developed and continually refined based upon scientific research conducted by The American Association of State Highway and Transportation Officials, which is the governing body for standards relating to design and construction of roadways. Thus, users can confidently implement changes to reduce an intersection’s crash frequency knowing their decisions are based on the best practices and knowledge of the industry.
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