Safety Data and Analysis in Developing Emphasis Area Plans (2008) / Chapter Skim
Currently Skimming:
Pages 28-41
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 28... ...
In addition, the data needed for the different roadway-segment-oriented crash types will differ and will be specified. The user is strongly urged to carefully review the material in each of the pertinent guides before beginning this planning process.
|
From page 29... ...
Exhibit IV1 will guide the user to the appropriate procedure. Procedure 1 – Choosing Roadway-Based Treatments and Target Populations When Treatment Effectiveness Is Known, and Both Crash and Non-Crash Data Are Available The following text identifies the data needed for conducting Procedure 1, followed by the individual steps in the procedure.
|
From page 30... ...
• A computerized crash data file which includes sufficient crash details to isolate target crash types (run-off-road, head-on crashes, and run-off-road on curves) , and potential target populations that will be affected by each treatment.
|
From page 31... ...
for each roadway class of interest. The "critical frequency" is the frequency of target crashes per mile that, if treated, will result in crash-injury reductions whose economic benefit will exceed implementation costs by some factor.
|
From page 32... ...
4. Link target crashes with roadway segments from the appropriate inventory data file, and then perform a computer screening of all segments on all routes that are potential treatment locations to determine which segments have crash frequencies that exceed the critical crash frequencies calculated above.
|
From page 33... ...
The above steps are then repeated for the second and subsequent potential treatment types. In each case, critical crash frequencies are calculated for each roadway class, the network screening program is used to identify treatment segments, and corrections are made for treatment gaps.
|
From page 34... ...
Since no detailed roadway inventory exists in this situation, these data will help in defining potential target locations within different roadway classes or types. • 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)
|
From page 35... ...
under consideration. As noted above under "Data Needs," the network screening program will need to output the total number of target crashes in the specified length, and the route number and mileposts for the first and last crash falling in each "window" for which the total number of target crashes exceeds the critical crash frequency.
|
From page 36... ...
As in Procedure 1, the above steps are then repeated for the second and subsequent potential treatment types. In each case, critical crash frequencies are calculated for each roadway class, the network screening is used to identify treatment segments, and corrections are made for treatment gaps.
|
From page 37... ...
The "critical frequency" is the annual frequency of target crashes per mile that, if treated, will result in crashinjury reductions whose economic benefit will exceed implementation costs by some factor. The same formula and information presented above under Step 2 of Procedure 1 will be used here.
|
From page 38... ...
While an alternative is to restrict the analysis to only fatal and serious-injury crashes, this will severely limit the crash sample, and will also omit a large component of the crash problem – non-serious injury and no-injury crashes. A better solution is to weight each crash by an economic cost based on its type and severity, and then accumulate the total crash cost within each target crash type.
|
From page 39... ...
The user may further refine this analysis by examining crash frequency or total crash cost within roadway classes. If the crash data are mileposted and linkable inventory data are available, details of roadway types can be linked to each crash record (e.g., number of lanes by divided/ undivided)
|
From page 40... ...
Total crash cost will be a much superior criterion if the target crash types being compared differ with respect to crash severity. If total crash cost or frequency for one treatment strategy clearly exceeds total crash cost or frequency for the other, the first would be a logical treatment choice.
|
From page 41... ...
b) When possible, formulate an effectiveness estimate that is applicable to particular target crash types only, not to total crashes.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.