Cover Image

Not for Sale

View/Hide Left Panel
Click for next page ( 48

The National Academies of Sciences, Engineering, and Medicine
500 Fifth St. N.W. | Washington, D.C. 20001

Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement

Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 47
47 to be applied to a given intersection. Since the combined inventory information or knowledge that will allow her/him effectiveness of two treatment strategies at the same lo- to determine which intersections are rural vs. urban, the cation will not be the simple sum of the two effectiveness number of lanes, the number of legs, and whether the road- levels, some correction must be applied for the second way is divided or undivided. This will be covered in the steps and all subsequent treatments that are applied to the of the procedure below.) The following are the specific data same intersection. Unfortunately, there is little knowl- needed to use Procedure 2A when choosing and targeting in- edge available about the combined effects of multiple tersection treatments. A description of each item is provided treatments. Until that knowledge is developed, it is sug- under Procedure 1 above and will not be repeated here. gested that the effectiveness level (Eff) of the second treatment applied to a given intersection be reduced to A specified effectiveness level (CRF or AMF) for each 50 percent of the level shown in the FHWA Sample Plan treatment to be examined for intersections (25), and the effectiveness of the third A computerized crash data file which includes sufficient treatment and subsequent treatments applied to the crash details to isolate target crash types (angle crashes, same segment be reduced to 25 percent of the level sideswipe crashes, run-off-road crashes, rear-end crashes, shown in the draft plan mentioned above. For example, and head-on crashes) and potential target populations assume that the first treatment for a given segment has an that will be affected by each treatment, and which is "mile- effectiveness level of 0.2, the second has an effectiveness posted" such that the location of each crash is included level of 0.15, and the third has an effectiveness level of As noted in the procedure below, it is also advantageous 0.10, and the fourth and subsequent treatments add no if the crash data contains information that can be used to additional effectiveness. The estimated combined effec- define "intersection class or type" e.g., information on tiveness of the three treatments applied to the same number of legs, rural vs. urban, type of traffic control, or segment would be 0.2 + 0.15(.5) + 0.1(.25) = 0.3. Again, any other intersection class characteristics. Since no de- this is only an estimate of the true combined effectiveness tailed intersection inventory exists in this situation, these at best. data will help in defining potential target locations within 9. Sum all expected crash injury reductions for all chosen different intersection classes or types. treatment types and chosen target locations and com- A network screening computer program which can read pare that total to the established goal. an input file composed of target crash records sorted by 10. Add new treatments, new targets, or new approaches route and milepost, and can count the number of target (e.g., inclusion of safety treatments in normal mainte- crashes within a given specified length (e.g., 1 mile) that nance or rehabilitation efforts) until the goal is met. have occurred in the past 3 to 5 years Again, the FHWA Sample Plan for intersections (25) Note that this program is different from the computer provides additional discussion of this option. program above. It is less likely to currently exist in a juris- diction, but can be developed by a knowledgeable com- puter analyst familiar with crash data files. Procedure 2A Choosing Intersection In general, the program will need to accumulate a count Treatments and Target Populations When each time a target (i.e., intersection-related) crash is found, Treatment Effectiveness Is Known and the milepost for each crash is reached, and the distance Mileposted Crash Data Are Available, but from the last crash (e.g., the difference between the two Detailed Inventory Data Are Not Available mileposts) is established for each adjacent pair of crashes. The following text identifies the data needed for conduct- The program will then accumulate the target crash count ing Procedure 2A, followed by the individual steps in the and cumulative distance until the distance is equal to or procedure. Note again that this procedure requires greater than the specified window length (e.g., 1 mile). If "mileposted" crash data. If mileposted data are not available, the accumulated distance is equal to 1 mile, which is un- refer to Procedure 2B or 3. likely, the program will output the number of target crashes and the milepost of the first and last crashes en- countered in that length and the distance between the first Data Needs and last crash; in this case, 1.0 mile. If the accumulated The data needed for Procedure 2A are virtually the same as distance is greater than 1 mile (i.e., the 1-mile limit fell be- for Procedure 1, except that neither detailed intersection in- tween two adjacent crashes), the counter should subtract ventory data nor linkable traffic counts are required. (Note the last crash added (i.e., the one outside the 1-mile limit), that this procedure operates more accurately if the user can and should again output the number of target crashes in not only sort crashes by route and milepost, but also has some the window, the milepost for the first crash and last crash

OCR for page 47
48 remaining in the window, and the distance between the The computer program will only detect intersections first and last crash which may be less than 1 mile in this that exceed the critical crash-frequency threshold. This case. A new window would then begin with the current will leave intersections that do not meet the threshold. The crash being considered (i.e., it would be counted and its user may desire to manually examine each of the intersec- milepost recorded) and the process would proceed until tions under consideration within each intersection class and the route ends. Each time the specified window length is determine whether or not these "below-threshold" inter- reached and output is produced, the count will be com- sections should be treated. The logical first answer is "no," pared to the critical frequency calculated in Step 2, and will since the intersections did not meet the critical threshold. only be retained in the final output file if the count exceeds However, there may be times when all or some of these in- the critical frequency. tersections should be included in the treatment program. Unit cost for each treatment both original implemen- 4. Estimate the expected crash/injury reductions on all the tation costs and annual maintenance costs identified target locations. Just as in Procedure 1, the results of this step will be used in Step 9 below in determining whether or not the Procedure goal is reached. Here, for each treatment intersection The general procedure for choosing and targeting treat- identified in Step 3, the user will need to determine the ments with known effectiveness levels was provided in Sec- number of crashes and injuries that will be reduced by this tion III above. The following text will expand that description treatment. This will be done by summing up all pertinent while focusing on intersection treatments those designed to crashes or crash injuries for all intersections to be treated, reduce intersection crashes. Because the user needs to under- and then multiplying this total by the estimated effective- stand the computerized procedure in order to input the ness level for the treatment under consideration. correct values and tailor it for his/her own jurisdiction, CI reduction = (CI at intersections) Eff the following provides the details of each step and sub-step in the procedure. Where: CI = "Goal-related" crashes or crash injuries 1. Develop critical crash frequencies for each candidate Eff = treatment effectiveness treatment type and intersection class of interest. The Just as in Procedure 1, the definition of "goal-related" "critical frequency" is the frequency of target crashes per crashes or injuries is, as implied, based on the nature of the intersection that, if treated, will result in crash-injury overall goal that has been established. If the goal is ori- reductions whose economic benefit will exceed imple- ented to fatal and injury target crashes, then these will be mentation costs by some factor. accumulated. If the goal is total target crashes, then these The same formula and information presented under will be accumulated. Step 2 of Procedure 1 above will be used here. The summing of goal-related crashes or injuries will be 2. Sort crashes by route and milepost in ascending order, done by estimating the annual number of such target and then perform a computer screening of all routes to crashes for all intersections selected for treatment. determine which locations (e.g., 1-mile "windows") 5. Repeat the above steps for each potential treatment type. have target crash frequencies that exceed the critical As in Procedure 1, the above steps are then repeated for crash frequencies calculated in Step 1. the second and subsequent potential treatment types. In This screening will be done using the network screening each case, critical crash frequencies are calculated for each program described above. If crash-based information is avail- intersection class and the computer program is used to able on intersection class or type (e.g., number of legs, urban identify treatment intersections. However, a final correc- vs. rural, type of traffic control), the target crash definitions tion is needed for intersections that have been identified should include these variables. The screening will then be for more than one treatment type, as detailed in the done independently for each of the intersection types (as de- following step. fined by crash variables) under consideration. As noted above 6. Correct for multiple treatments at the same intersection. under "Data Needs," the network screening program will The user will again need to correct for multiple treat- need to output the total number of target crashes in the spec- ments at the same intersection. The same rationale and ified length and the route number and mileposts for the first procedure followed in Procedure 1 will be followed here. and last crash falling in each "window" for which the total (See Step 8 of Procedure 1). number of target crashes exceeds the critical crash frequency. 7. Sum all expected crash injury reductions for all chosen 3. Review the computer output for "below-threshold" treatment types and chosen target locations and com- intersections. pare that total to the established goal.