Safety Data and Analysis in Developing Emphasis Area Plans (2008) / Chapter Skim
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Pages 16-27
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From page 16... ...
As detailed below in Section III, Stage 3 of this guide, one method of combining both frequency and severity is through weighting each crash in each crash type by an economic cost based on its severity. Information on economic cost per crash by severity level for 22 different crash types categorized by speed limit category can be found in Crash Cost Estimates by Maximum PoliceReported Injury Severity Within Selected Crash Geometries (22)
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From page 17... ...
The revised process is much more iterative and analysis-driven where initial goals are modified based on analyses that indicate what is realistic given the nature and size of the problem, the known or assumed countermeasure effectiveness for the final list of chosen countermeasures, and the optimization of the existing budget either within a given emphasis area or across emphasis areas. Stage 3 – Define Treatment Strategies and Target Populations Having now defined the issue/emphasis area to be treated (e.g., run-off-road crashes, crashes involving drinking drivers)
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From page 18... ...
. Procedure 1 is suitable for roadway-based treatments that address specific crash types and are aimed at modifying roadway segments or intersections.
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From page 19... ...
for each treatment to be considered • A computerized crash data file which includes sufficient crash details to identify crash types that will be affected by each treatment ("target crashes" such as run-off-road crashes, head-on crashes, run-off-road on curves) , and which includes crashes for all potential target populations • Computerized roadway inventory data and/or intersection inventory data that can be linked to the crash data by location of the crash • A network screening computer program which will examine each segment of roadway of a given length (e.g., 1 mile)
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From page 20... ...
Procedure 2 – Choosing Roadway-Based Treatments and Target Populations When Treatment Effectiveness Is Known and Crash Data Are Available, but Detailed Inventory Data Are Not Available Procedure 2 for roadway-based treatments is intended for application on roadway segments or at intersections in "limited-data" situations. Here, crash data are required, but a formal roadway segment or intersection inventory database is not required.
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From page 21... ...
and potential target populations that will be affected by each treatment, and which is "mileposted" such that the location of each crash is included • A network screening computer program which can read an input file composed of target crash records sorted by route and milepost, and can count the number of target crashes within a given specified length (e.g., 1 mile for segmentbased treatments and 500 ft for intersection treatments) that have occurred in the past 3 to 5 years • Unit cost for each treatment – both original implementation costs and annual maintenance costs If these data are available, Procedure 2A will lead the user through a series of steps that will allow the user to choose a set of treatments and a set of targets for each chosen treatment that will both meet the established crash/injury reduction goal, and will be cost-beneficial at some prescribed level.
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From page 22... ...
As with Procedure 1, one must analyze each potential treatment separately (choosing targets for each) , implement a procedure which estimates "combined effectiveness" for segments or intersections where multiple treatments for the same crash types will be applied, and sum potential crash injury reductions across all treatments to determine if the established goal can be met.
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From page 23... ...
The analysis of treatments without documented crash/ injury reduction effectiveness should focus on those treatments for which the user decides that there is some evidence of a crash/injury reduction benefit, even if a documented CRF or AMF based on research using crash data is not available. The NCHRP Report 500 guide series classifies treatments into three types – proven, tried, and experimental – based on whether credible evaluations based on crash data have been performed (i.e., a CRF or AMF has been defined)
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From page 24... ...
Thus, if one is attempting to reduce older driver crashes, then a preliminary analysis should be performed using available crash data to determine the frequencies and proportions of different types of crashes involving older drivers. Then, treatments that target the most prevalent crash types should be selected.
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From page 25... ...
Cc* Eff Where: • N = Number of target crashes for the subpopulation or location where the treatment is to be applied • Cc = average economic cost per target crash • Eff = treatment effectiveness, or the percent reduction in target crashes Since different severity levels have different crash costs, the value used for Cc can be a weighted average of the crash costs associated with the crash types likely to be affected.
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From page 26... ...
b) When possible, formulate an effectiveness estimate that is applicable to particular target crash types only, not to total crashes.
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From page 27... ...
using the guidance provided earlier and guidance provided in other documents such as NCHRP Report 501. The following sections of this manual will provide more detail on how to conduct the final step – treatment selection and targeting – for each of the 22 emphasis areas in the AASHTO Strategic Highway Safety Plan, or for groups of emphasis areas for which the data needed and the procedures to be applied are similar.
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