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34 tional effectiveness. The estimated combined effectiveness As noted in the procedure below, it is also advantageous of the three treatments applied to the same segment would if the crash data contains information that can be used to be 0.2 + 0.15(.5) + 0.1(.25) = 0.3. Again, this is only an es- define "roadway class or type" e.g., information on num- timate of the true combined effectiveness at best. ber of lanes, route type, divided vs. undivided, or any other 9. Sum all expected crash injury reductions for all chosen roadway class characteristics. Since no detailed roadway treatment types and chosen target locations and com- inventory exists in this situation, these data will help in pare that total to the established goal. defining potential target locations within different road- 10. Add new treatments, new targets or new approaches way classes or types. (e.g., inclusion of safety treatments in normal mainte- A network screening computer program which can read nance or rehabilitation efforts) until the goal is met. an input file composed of target crash records sorted by Again, the example draft plan presented at the FHWA route and milepost, and can count the number of target web site ( crashes within a given specified length (e.g., 1 mile) that lanedeparture/index.htm) provides additional discussion of have occurred in the past 3 to 5 years. this option. Note that this program is different from the network screening program in Procedure 1, because it is applied to mileposted crash data for which no roadway-segment-based Procedure 2A Choosing Roadway-Based inventory data is available. If such a program does not cur- Treatments and Target Populations rently exist in a jurisdiction, it can be developed by a knowl- When Treatment Effectiveness Is Known edgeable computer analyst familiar with crash data files. and Mileposted Crash Data Are Available, In general, the program will need to accumulate a count but Detailed Inventory Data Are Not each time a target crash is found, the milepost for each crash Available reached, and the distance from the last crash (e.g., the differ- The following text identifies the data needed for conduct- ence between the two mileposts) for each adjacent pair of ing Procedure 2A, followed by the individual steps in the crashes. The program will then accumulate the target crash procedure. Note again that this procedure requires count and cumulative distance until the distance is equal to "mileposted" crash data. If mileposted crash data are not or greater than the specified window length (e.g., 1 mile). If available, refer to Procedure 2B or 3. the accumulated distance is equal to 1 mile, which is un- likely, the program will output the number of target crashes and the milepost of the first and last crashes encountered in Data Needs that length and the distance between the first and last crash, The data needed for Procedure 2A are virtually the same as in this case, 1.0 mile. If the accumulated distance is greater for Procedure 1, except that neither detailed roadway inventory than 1 mile (i.e., the 1-mile limit fell between two adjacent data nor linkable traffic counts are required. (Note that this pro- crashes), the counter should subtract the last crash added cedure operates more accurately if the user can not only sort (i.e., the one outside the 1-mile limit), and should again crashes by route and milepost, but also has some inventory output the number of target crashes in the window, the information or knowledge that will allow him/her to determine milepost for the first crash and last crash remaining in the which route-milepost ranges are rural vs. urban, the number of window, and the distance between the first and last crash, lanes, and whether the roadway is divided or undivided. This which may be less than 1 mile in this case. A new window will be covered in the steps of the procedure below.) The would then begin with the current crash being considered following are the specific data needed to use Procedure 2A when (i.e., it would be counted and its milepost recorded) and the choosing and targeting roadway-based treatments. A descrip- process would proceed until the route ends. Each time the tion of each item is provided under Procedure 1 above and will specified window length is reached and output is produced, not be repeated here. the count will be compared to the critical frequency calcu- lated in Step 2, and will only be retained in the final output A specified effectiveness level (CRF or AMF) for each file if the count exceeds the critical frequency. treatment to be examined. Unit cost for each treatment both original implementa- A computerized crash data file that includes sufficient tion costs and annual maintenance costs crash details to isolate target crash types (run-off-road, head-on crashes, run-off-road on curves, etc.) and poten- Procedure tial target populations that will be affected by each treat- ment, and which is "mileposted" such that the location of The general procedure for choosing and targeting treat- each crash is included. ments with known effectiveness levels was provided in Section

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35 III above. The following text will expand that description related will lead to acceptable results in this screening ef- while focusing on roadway-segment treatments those de- fort, even for shorter than maximum-length curves. signed to reduce lane departure crashes. Because the user 3. Correct the output for "treatment gaps" along the same needs to understand the computerized procedure in order to route resulting from the network screening computer input the correct values and tailor it for their own jurisdiction, program. the following provides the details of each step and sub-step. If this step is done, the user will follow a "manual re- view" procedure similar to that described under Step 5 of 1. Develop critical crash frequencies for each candidate Procedure 1. The rationale for this step is provided there. treatment type and roadway class of interest. The "criti- As before, this correction will require that the user manu- cal frequency" is the frequency of target crashes per mile ally examine each of the routes under consideration within that, if treated, will result in crash-injury reductions each roadway class to detect possible treatment gaps. Here, whose economic benefit will exceed implementation the user will take the output of the network screening costs by some factor. program (i.e., the route and locations of the originally The same formula and information presented under chosen treatment sites), enter these sites into some type of Step 2 of Procedure 1 above will be used here. spreadsheet (one treatment window per row), and then However, if the user is attempting to target horizontal sort the rows by route and beginning milepost. By scan- curve treatments without an inventory file that defines the ning down this listing for each route under consideration, beginning and ending mileposts for each curve, a different the user can determine where the treatment gaps are "annual cost per unit length" will have to be used. Here, located along each route the missing segments in the the user will have to estimate the average length of curve listing. (Note that some of these missing segments would in his jurisdiction, and will then estimate the annual treat- result from the fact that the roadway class changed within ment cost of that length. It is suggested that if there are dif- the route e.g., from two-lane to four-lane. This determi- ferent types of terrain in a jurisdiction (e.g., mountainous nation will have to be made based on the user's knowledge vs. level terrain) that would significantly affect the average of where the roadway class changes along a given route.) curve length, the user would estimate an average length However, because there is no inventory data available and cost for curves in each terrain type. under this procedure, the user will not have inventory data 2. Sort crashes by route and milepost in ascending order, that provides detailed information on the AADT levels and then perform a computer screening of all segments and roadway characteristics of the treatment gaps adjacent on all routes that are potential treatment locations to de- to the identified treatment windows. This determination termine which segments have target crash frequencies will have to be based on the user's knowledge of the route that exceed the critical crash frequencies calculated in in question. If such knowledge is available, the user will Step 2. identify additional, similar treatment segments for each This screening will be done using the network screen- treatment/road class combination. ing program described above. If crash-based information 4. Estimate the expected crash injury reductions on all the is available on roadway class or type (e.g., number of lanes, identified target locations. route type, divided vs. undivided), the target crash defini- Just as in Procedure 1, the results of this step will be used in Step 8 below in determining whether or not the tions should include these variables (e.g., run-off-road goal is reached. Here, for each treatment window identi- crashes on two-lane roads and head-on crashes on divided fied at the end of Step 3 (i.e., after correction for treatment roads). The screening will then be done independently for gaps), the user will need to determine the number of each of the roadway types (as defined by crash variables) crashes and injuries that will be reduced by this treatment. under consideration. As noted above under "Data Needs," This will be done by summing up all pertinent crashes or the network screening program will need to output the crash injuries for all windows to be treated, and then mul- total number of target crashes in the specified length, and tiplying this total by the estimated effectiveness level for the route number and mileposts for the first and last crash the treatment under consideration. falling in each "window" for which the total number of target crashes exceeds the critical crash frequency. CI reduction = (CI on segments) Eff Note that if the treatment being considered is for hori- Where: zontal curves (for example, the user is searching for a CI = Goal-related crashes or crash injuries "system" of horizontal curves to correct with improved Eff = treatment effectiveness curve warnings), the window length should be defined as the expected maximum (not average) curve length in a Just as in Procedure 1, the definition of "goal-related" given terrain. The use of target crashes that are only curve- crashes or injuries is, as implied, based on the nature of the