National Academies Press: OpenBook

Safety Data and Analysis in Developing Emphasis Area Plans (2008)

Chapter: Section V - Roadway Junctions

« Previous: Section IV - Roadway Segment Programs
Page 42
Suggested Citation:"Section V - Roadway Junctions." National Academies of Sciences, Engineering, and Medicine. 2008. Safety Data and Analysis in Developing Emphasis Area Plans. Washington, DC: The National Academies Press. doi: 10.17226/14170.
×
Page 42
Page 43
Suggested Citation:"Section V - Roadway Junctions." National Academies of Sciences, Engineering, and Medicine. 2008. Safety Data and Analysis in Developing Emphasis Area Plans. Washington, DC: The National Academies Press. doi: 10.17226/14170.
×
Page 43
Page 44
Suggested Citation:"Section V - Roadway Junctions." National Academies of Sciences, Engineering, and Medicine. 2008. Safety Data and Analysis in Developing Emphasis Area Plans. Washington, DC: The National Academies Press. doi: 10.17226/14170.
×
Page 44
Page 45
Suggested Citation:"Section V - Roadway Junctions." National Academies of Sciences, Engineering, and Medicine. 2008. Safety Data and Analysis in Developing Emphasis Area Plans. Washington, DC: The National Academies Press. doi: 10.17226/14170.
×
Page 45
Page 46
Suggested Citation:"Section V - Roadway Junctions." National Academies of Sciences, Engineering, and Medicine. 2008. Safety Data and Analysis in Developing Emphasis Area Plans. Washington, DC: The National Academies Press. doi: 10.17226/14170.
×
Page 46
Page 47
Suggested Citation:"Section V - Roadway Junctions." National Academies of Sciences, Engineering, and Medicine. 2008. Safety Data and Analysis in Developing Emphasis Area Plans. Washington, DC: The National Academies Press. doi: 10.17226/14170.
×
Page 47
Page 48
Suggested Citation:"Section V - Roadway Junctions." National Academies of Sciences, Engineering, and Medicine. 2008. Safety Data and Analysis in Developing Emphasis Area Plans. Washington, DC: The National Academies Press. doi: 10.17226/14170.
×
Page 48
Page 49
Suggested Citation:"Section V - Roadway Junctions." National Academies of Sciences, Engineering, and Medicine. 2008. Safety Data and Analysis in Developing Emphasis Area Plans. Washington, DC: The National Academies Press. doi: 10.17226/14170.
×
Page 49
Page 50
Suggested Citation:"Section V - Roadway Junctions." National Academies of Sciences, Engineering, and Medicine. 2008. Safety Data and Analysis in Developing Emphasis Area Plans. Washington, DC: The National Academies Press. doi: 10.17226/14170.
×
Page 50
Page 51
Suggested Citation:"Section V - Roadway Junctions." National Academies of Sciences, Engineering, and Medicine. 2008. Safety Data and Analysis in Developing Emphasis Area Plans. Washington, DC: The National Academies Press. doi: 10.17226/14170.
×
Page 51
Page 52
Suggested Citation:"Section V - Roadway Junctions." National Academies of Sciences, Engineering, and Medicine. 2008. Safety Data and Analysis in Developing Emphasis Area Plans. Washington, DC: The National Academies Press. doi: 10.17226/14170.
×
Page 52
Page 53
Suggested Citation:"Section V - Roadway Junctions." National Academies of Sciences, Engineering, and Medicine. 2008. Safety Data and Analysis in Developing Emphasis Area Plans. Washington, DC: The National Academies Press. doi: 10.17226/14170.
×
Page 53

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

42 Planning Programs Related to Reducing Crashes at Signalized and Unsignalized Intersections This section of the guide provides the details of the four levels of treatment choice and targeting procedures described above in the Stage 3 discussion in Section III, but it is oriented to those issues/emphasis areas that are specifically related to at- grade intersections – angle crashes, turning crashes, sideswipe crashes, rear-end crashes, head-on crashes and run-off-road crashes. In most instances, a given procedure will follow the same basic steps, regardless of the crash type being addressed. Where the procedure differs between crash types, this will be noted. In addition, the data needed for the different intersection-oriented crash types will differ and will be spec- ified. The user is strongly urged to carefully review the ma- terial in each of the pertinent guides before beginning this planning process. These intersection-oriented guides are found within NCHRP Report 500: Guidance for Implemen- tation of the AASHTO Strategic Highway Safety Plan. The specific volumes pertinent to this section are: • Volume 5: A Guide for Addressing Unsignalized Intersection Collisions (5) • Volume 12: A Guide for Reducing Collisions at Signalized In- tersections (12) A link to these downloadable guides can be found at http:// safety.transportation.org/guides.aspx. Possible Program Types – Spot versus System Programs Before moving to the specific treatment choice/targeting procedures for these emphasis areas, it is noted that states who were early participants in the AASHTO safety planning process for intersection programs started from two different perspectives, and the perspective chosen determines the choice and targeting of treatments. Some states chose to try to expand their current “high-crash location (HCL)” pro- gram to include more locations to meet their overall goal. Others chose to orient their planning methods to the identi- fication and treatment of “systems” of intersections, not just those locations that fell under the HCL program. Indeed, guidance provided in each of the guides, in companion train- ing courses, and in the FHWA Sample Plan for intersections (25) is that system-based programs will need to be included. If the jurisdiction is really attempting to reach a goal which represents a significant change from the current situation – a stretch goal – it is very unlikely that expansion of the HCL program will suffice. While such an expansion is clearly a component of a stretch-goal plan, large-scale treatment of systems and corridors will also likely be necessary. Indeed, a jurisdiction can use the following procedure to determine approximately how much the existing HCL program will have to be expanded, which will provide some guidance on whether system programs should also be considered. 1. Examine the most recent listing of HCL projects that were chosen for treatment in your jurisdiction and identify those that were related to intersection crashes. 2. Add the numbers of before-treatment crashes, injuries, and fatalities from each intersection and divide by the number of years of before data to produce a total number of potentially treatable intersection crashes and crash injuries per year. 3. Multiply these totals by 20 percent to get the number of intersection crashes, injuries and fatalities that are expected to be reduced per year by your current program. (This assumes an average Crash Reduction Factor of 20 percent for all intersection strategies. This is probably too high, but in the ballpark of reality, and good enough for this exercise.) S E C T I O N V Roadway Junctions

4. Compare these numbers of crashes and injuries reduced and lives saved to your statewide intersection goal and calculate the proportion of your total goal that this represents. 5. To calculate approximately how much you will have to expand the intersection part of your HCL program to meet your goal, divide 1.0 by the proportion from the pre- vious step. For example, if the fatality and injury savings from your current program is 20 percent of your goal, then you will have to identify and treat five times as many intersections in the future (i.e., 1.0 / 0.20 = 5). The user will then need to make the determination of whether enough sites with high numbers of intersection crashes can be identified. Usually the HCL program has identified more in- tersections that can be treated. This full “census” of potential HCL sites can be examined to determine whether enough sites with high numbers of intersection crashes are available. In most cases, if a stretch goal has been set, the answer will be “no.” In that case, the user should consider adding system im- provements to the plan. While all states and some local jurisdictions have proce- dures in place to identify and treat high-crash locations, it is noted that an improved methodology is currently being de- veloped by FHWA in the SafetyAnalyst program described in the preceding section (also see http://www.safetyanalyst.org). This set of the software tools for safety management of spe- cific highway sites includes a series of procedures that will allow the user to identify high-crash locations or sites with potential for safety improvement, diagnose potential treat- ment sites to identify correctable crash patterns, conduct an economic analysis to assure a minimum B/C ratio, and de- velop a combined treatment program which maximizes the benefits that can be gained from a given total treatment budget. The network-screening tools within SafetyAnalyst provide a good approach for applying Procedure 1. If preliminary analysis indicates that even an enhanced and expanded high-crash location program will not meet the goal, then the users will need to add systems-based treatment programs to the effort. Indeed, the four procedures de- scribed earlier and detailed below are developed to assist the user in identifying intersections to treat and to help define the treatments that should be applied to them. Again, the choice between which procedure is appropriate is defined by three factors – whether or not treatment effectiveness is known, whether the jurisdiction has inventory data that can be linked to their crash data, and whether the crashes are “mileposted” or not. Exhibit V-1 will guide the user to the appropriate procedure. Procedure 1 – Choosing Intersection 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 con- ducting Procedure 1, followed by the individual steps in the procedure. Data Needs The following are the specific data needed to use Procedure 1 when choosing and targeting intersection treatments. • A specified effectiveness level (CRF or AMF) for each treatment to be examined The “Treatment Effectiveness” section under each treat- ment in each NCHRP Report 500 guide provides a descrip- tion of what is known about CRFs. It is important that the user review the material in the guides for a given treatment. Valuable information about the stability of the CRF, cau- tions about the use of the treatment and other essential- information is included there, but will not be repeated here. For a significant proportion of the treatment strategies defined in the six guides covered in this section, a specific AMF is not presented. Since the preparation of some of the earlier guides, additional information on treatment CRFs has been developed in both NCHRP Project 17-25, and in preliminary work for the Highway Safety Manual. The AMFs from NCHRP Project 17-25 have been published in NCHRP Research Results Digest 299 (27). The AMFs devel- oped in NCHRP Project 17-27 will be incorporated in the forthcoming Highway Safety Manual. 43 Inventory Data Available and Linkable to Crashes? Yes No Treatment Effectiveness Known? Mileposted Crashes Mileposted Crashes Unmileposted Crashes Yes Procedure 1 Procedure 2A Procedure 2B No Procedure 3 Procedure 3 Procedure 3 Some known, some unknown Procedure 4 Procedure 4 Procedure 4 Exhibit V-1. Guide to choice of procedures based on knowledge of treatment effectiveness and crash data quality.

• A computerized crash data file which includes sufficient crash details to isolate target crash types (angle, sideswipe, turning, rear-end, and head-on crashes), and potential tar- get populations that will be affected by each treatment Here, the user will need to examine the data formats for variables in their crash file to identify variables and codes within variables that can be used in determining whether or not each crash in the file is a “target crash.” Crash databases often categorize crash data for a given crash into up to three subfiles – general accident/crash variables, variables for each vehicle in the crash, and variables for each occupant in the crash. In general, the analyst will need to first screen to de- termine if the crash is an intersection or non-intersection crash, and then examine the different crash types within those that are intersection-related. Since “intersection crashes” can include both those in the intersection itself and on the intersection approaches (e.g., rear-end crashes), the analyst will usually have to include those crashes that are coded as both “intersection” and “intersection-related.” The variables needed to determine whether a crash is a “target crash” or not, for intersection crashes, can usually be found in one of the first two subfiles – crash or vehicle data. Crash files differ from jurisdiction to jurisdiction. While certainly not always the case, the following variables (or similar vari- ables) listed in Exhibit V-2 will be used in this determination. • Computerized intersection inventory data that can be linked to the crash data by location of the crash A few state DOTs have computerized intersection in- ventory files for the full state highway system that can be linked to crashes, since both the intersections in the inven- tory file and the crashes are identified by “addresses” – usu- ally route and milepost or GIS coordinates. Most local ju- risdictions (i.e., counties, towns, townships, and cities) do not have such an inventory system. For jurisdictions that do not have an inventory file, Procedures 2A and 2B below can be used. A recommended set of data elements for inclusion in intersection inventories is under development by FHWA for the Minimum Inventory of Roadway Elements (MIRE). • A computer program that will examine each intersection and calculate the number of target crashes (e.g., angle, turning, sideswipe, run-off-road crashes, rear-end, and head-on) that have occurred at each intersection in the past 3 to 5 years. This program exists in some jurisdictions, but may not exist in others. If not, a knowledgeable computer analyst can build one. The program will require that counts of target crashes can be made and “attached to” each inter- section within an agency’s jurisdiction (or this could be done for all of the intersections on a particular route). The program must then be able to examine each inter- section and record the total number of crashes in an out- put file, along with the route number and an intersection identifier. The process would be repeated for all routes in the system. • Computerized traffic count data that can be linked to the intersection inventory data (unless it is contained in that database). While the procedure can be operated without comput- erized count data, these data are often available in state DOT files that have an intersection inventory system. This is not always the case in urban systems. If available, the traffic count information can be used to further target the potential treatment sites in two ways. First, if the user only wishes to treat “high-volume” intersections, these data can be used to screen out “low-volume” intersections prior to running the computer program. Second, after the program has been run, the identified sites can be further screened by a given AADT level, or the sites can be sorted by AADT to assist the user in final site choice. • Unit cost for each treatment – both original implementa- tion costs and annual maintenance costs The NCHRP Report 500 guides do not provide treatment costs due to differences between states and expected changes over time. The user will need to obtain information 44 Crash Type Variable Crash Database Subfile Intersection or Intersection- related Relation to Junction Location Type Crash Angle Crashes Accident/Crash Type Sequence of Events First Harmful Event Most Harmful Event Crash Crash or Vehicle Crash Vehicle Left- and Right-turning Crashes Same as Angle Same as Angle Sideswipe Crashes Same as Angle Same as Angle Run-Off-Road Crashes Same as Angle plus Number of Vehicles or Units Crash Rear-end Crashes Same as Angle plus Number of Vehicles or Units Crash Head-On Crashes Same as Angle Same as Angle Exhibit V-2. Crash variables and subfile location by crash type.

on such costs, either from vendors or from other jurisdic- tions that have used the treatment. The guides do provide “Information on Agencies or Organizations Currently Im- plementing This Strategy” that could be contacted for help under each of the treatment strategies. (The guides provide only early users, and surrounding jurisdictions may have implemented the treatment after the guide was completed.) Finally, the user will need an estimate of annual mainte- nance cost (per intersection or intersection approach) for each treatment to be analyzed. Since maintenance includes “replacement after a crash” in some cases, the user may have to make estimates of the number of expected crashes per year and the amount of expected damage. Again, past users of the treatment can be of assistance here. Procedure The general procedure for choosing and targeting treat- ments with known effectiveness levels was provided in Section III above. The following text will expand that description while focusing on intersection treatments designed to reduce intersection-related crashes. Because the user needs to under- stand the computerized procedure in order to input the correct values and tailor it for their own jurisdiction, the following provides the details of each step and sub-step. 1. Specify the types/classes of intersections that are poten- tial targets for the treatments. Because the choice of treatments, the treatment ef- fectiveness, and the treatment cost per intersection may differ by intersection class/type, the user will need to specify the types/classes of interest – e.g., four-leg rural intersections, three-leg urban intersections, rural unsignalized intersections, urban signalized intersec- tions. If desired, these potential treatment sites could be further screened by AADT level (e.g., only “high- volume” intersections). 2. Develop critical crash frequencies for each candidate treatment type (e.g., left-turn lane) for each intersection class of interest. The “critical frequency” is the fre- quency of target crashes per intersection that, if treated, will result in crash-injury reductions whose economic benefit will exceed implementation costs by some fac- tor. In the FHWA Sample Plan for intersections (25), the target benefit to cost ratio used was 2.0 or greater. These “critical frequencies” must be developed for each candidate treatment being examined. If the same treatment is to be used on different intersection classes, it will be nec- essary to develop different critical frequencies for each treatment by intersection class if the treatment cost per unit length or treatment effectiveness varies by intersection class. The following formula is used: CF = (Ann. Cost)(Target B/C)/(Eff)(Avg. Crash Cost) Where: • CF = Critical annual frequency of target crashes per in- tersection to consider the strategy to be cost effective. • Ann. Cost = The annual cost of the improvement per intersection. If it is a construction improvement, it is the construction costs annualized over the expected life of the improvement. • Target B/C = The B/C ratio defined by the user. It is usually between 1.0 and 2.0. In the FHWA Sample Plan for intersections (25), a value of 2.0 is used. • Eff = The estimated effectiveness of the treatment strategy in reducing targeted crashes, expressed as a proportion (i.e., the CRF/100). This can be extracted from NCHRP Research Results Digest 299 (27) or from other sources. • Avg. Crash Cost = The average economic cost per crash for the target crash type that will be affected by this treatment strategy. The following estimates were based on costs from Table 10 of Crash Cost Estimates by Maximum Police-Reported Injury Severity Within Selected Crash Geometries (22), and represent compre- hensive costs in terms of 2001 dollars. Comprehensive cost estimates include not only the monetary losses as- sociated with medical care, other resources used, and lost work, but also non-monetary costs related to the reduction in the quality of life. The cost for each crash type is shown in Exhibit V-3 for two ranges of speed limits: ≤ 45 mph and ≥ 50 mph. The former should be useful for urban crashes, and the latter for rural crashes. 3. Using the inventory file, stratify potentially treatable intersections by intersection class. This stratification will result in a file of intersections for each of the intersection classes under consideration for each treatment. 4. Link target crashes with intersections from the appro- priate inventory data file, and then perform a computer screening of all intersections that are potential treat- ment locations to determine which intersections have crash frequencies that exceed the critical crash fre- quencies calculated above. This will be done using the computer program de- scribed above, and will be done independently for each of the intersection types under consideration. Note that if the treatment being considered is for three-leg intersections (i.e., the user is searching for a “system” of three-leg intersections to correct with, say, left-turn lanes), this step will require that the user’s in- tersection inventory system can identify the locations of three-leg intersections. If no such data are available, then 45

the user will have to use either Procedure 2A or 2B in- stead of this Procedure 1. 5. Review the computer output for “below-threshold” intersections. The computer program will only detect intersections that exceed the critical crash-frequency threshold. This will leave intersections that do not meet the threshold. The user may desire to manually examine each of the in- tersections under consideration within each intersection class and determine whether or not these “below-thresh- old” intersections should be treated. The logical first an- swer is “no,” since the intersections did not meet the crit- ical threshold. However, there may be times when all or some of these intersections should be included in the treatment program. 6. Estimate the expected crash/injury reductions on all the identified target locations. The results of this step will be used in Step 9 below in determining whether or not the goal is reached. Here, for each treatment location within a given intersection class, the user will need to determine the number of crashes and injuries that will be reduced by this treatment. This will be done by summing up all pertinent crashes or crash injuries for all intersections to be treated, and then mul- tiplying this total by the estimated effectiveness level for the treatment under consideration. CI reduction = (CI at intersections) × Eff Where: CI = “Goal-related” crashes or crash injuries Eff = treatment effectiveness The definition of “goal-related” crashes or injuries is, as implied, based on the nature of the overall goal that has been established. If the goal is oriented to fatal and injury target crashes, then these will be accumulated. If the goal is total target crashes, then these will be accumulated. The summing of goal-related crashes or injuries will be done by using a computer program to estimate the an- nual number of such target crashes for all intersections selected for treatment. Users with full crash and inven- tory systems who have developed the [computer] pro- gram will have the ability to link such goal-oriented tar- get crashes to each intersection selected and to sum the total over all intersections. The best annual estimate will be one based on more than 1 year of past data (3 to 5 years) and then dividing by the number of years used. 7. Repeat the above steps for each potential treatment type. The above steps are then repeated for the second and subsequent potential treatment types. In each case, criti- cal crash frequencies are calculated for each intersection class, the computer program is used to identify treatment intersections, and decisions are made whether to treat any “below-threshold” intersections. However, a correc- tion is needed for intersections that have been identified for more than one treatment type, as detailed in the following step. 8. Correct for multiple treatments on the same intersection. Since many intersection treatments affect the same type of target crash, the above procedure will identify the same intersection as a potential for treatment in many cases. In these cases, the user has two options: (1) choose only one treatment for each of these intersections, or (2) choose to implement two or more treatments at the same intersection. Under Option 1, the user would compare the lists of potential treatment intersection sites from Step 5 above, and would decide which treatment to place at each inter- section where two or more treatments could be imple- mented. That intersection (and its related goal-oriented crashes or injuries) is then removed from the list of inter- sections for all other treatments. Under Option 2, the user must develop some measure of combined effectiveness for the two or more treatments 46 Crash Type Traffic Control Speed Limit Category Comprehensive Cost/Crash* < 45 mph $22,000 Signalized > 50 mph $75,000 < 45 mph $32,000 Angle/turning Crashes Unsignalized > 50 mph $96,000 < 45 mph $16,000 Sideswipe Crashes Signalized or Unsignalized > 50 mph $55,000 < 45 mph $24,000 Rear-end Crashes Signalized or Unsignalized 50 mph $33,000 < 45 mph $16,000 Head-On Crashes Signalized or Unsignalized > 50 mph $88,000 < 45 mph $67,000 Run-Off-Road Fixed Object Crashes Signalized or Unsignalized > 50 mph $107,000 * Cost in 2001 dollars (22) > Exhibit V-3. Crash cost by crash type, traffic control and posted speed limit.

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

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

8. Add new treatments, new targets, or new approaches (e.g., inclusion of safety treatments in normal mainte- nance or rehabilitation efforts) until the goal is met. Again, the FHWA Sample Plan for intersections (25) provides additional discussion of this option. Procedure 2B – Choosing Intersection Treatments and Target Populations When Treatment Effectiveness Is Known and Neither Mileposted Crash Data nor Detailed Inventory Data Are Available The following text identifies the data needed for conduct- ing Procedure 2B, followed by the individual steps in the procedure. Note again that since no mileposted crash data exists, this procedure will only allow the user to identify intersection types within a given jurisdiction as potential treatment sites, but not specific intersection locations. It will also not allow the user to target the treatments to specific locations along the route. Data Needs The data needed for Procedure 2B are less than required in either of the previous two procedures. Major differences in- clude the fact that no inventory or traffic data are required, and that the crashes do not have to be “mileposted” to a spe- cific location on a specific route. However, each crash record must contain information on the county or local jurisdiction where the crash occurred, along with the name of the route/street where the crash occurred. • A specified effectiveness level (CRF or AMF) for each treatment to be examined. • A computerized crash data file which includes suffi- cient crash details to isolate crash types that will be affected by each treatment (“target crashes” – e.g., angle crashes, sideswipe crashes, run-off-road crashes, rear- end crashes, and head-on crashes), which includes crashes for all potential target populations. Each crash record must contain a county or jurisdiction name where the crash occurred, and the name of the route or road where the crash occurred. • Route length information that will provide the length in miles of each road or route within a county or local juris- diction that is a potential target for any treatment, or at least the approximate length. If this information is not available in the user’s files, it may be available from other sources including road maintenance records, and can be scaled from maps if necessary. • Unit cost for each treatment – both original implemen- tation costs and annual maintenance costs. See discussion under Procedure 1. Procedure The following steps are those that would be followed for choosing among intersection treatments with known effec- tiveness and targeting these treatments to entire intersection classes in a jurisdiction. 1. Develop critical crash frequencies for each candidate treatment type and intersection class of interest. The “critical frequency” is the annual frequency of tar- get crashes per mile that, if treated, will result in crash- injury reductions whose economic benefit will exceed implementation costs by some factor. The same formula and information presented under Step 2 of Procedure 1 above will be used here. 2. Link target crashes to each route in each jurisdiction (but not to a specific point on the route). This will require computer sorting of crashes by each named route. Some manual effort will be required to correct misspelled names and to group routes or streets that have multiple names. The output of this program will be a listing of target crashes sorted by route name. Note that multiple years of crashes can be used, and in- deed the procedure will be more accurate if more than 1 year’s crash data (e.g., 3 to 5 years) are used. If multiple years are used, there may be situations where a route was renamed during the period. If so, both crashes with the original and new name should be accumulated under one route name. 3. Develop a spreadsheet that contains the count of target crashes for each route (one route per row), along with the mileage for that route. It may be possible for the computer program used to sort the crashes in Step 2 to output this count for each route. If not, the counts can be made manually. The final output of this step is a spreadsheet containing a total count of target crashes and the length in miles for each route under consideration. 4. Calculate the annual crash frequencies per mile for each potential route. If a spreadsheet is used, this is a simple step in which the crash count is divided by the route length times the number of years of crashes used. 5. Identify routes to be treated by determining which have calculated annual frequencies per mile that exceed the developed critical crash frequencies per mile. This is a comparison of the output of Step 4 with the “critical frequencies” defined in Step 1. 49

6. Estimate the expected crash injury reductions on all the identified target routes. Just as in Procedure 1, the results of this step will be used in Step 9 below in determining whether or not the goal for the jurisdiction is reached. Here, for each treat- ment route identified at the end of Step 5, the user will need to determine the annual number of crashes and in- juries that will be reduced by this treatment. This will be done by summing up all pertinent crashes or crash in- juries for all routes to be treated, and then multiplying this annual total by the estimated effectiveness level for the treatment under consideration. CI reduction = (CI) × Eff Where: CI = Annual “goal-related” crashes or crash injuries on the routes chosen Eff = treatment effectiveness Just as in Procedure 1, the definition of “goal-related” crashes or injuries is, as implied, based on the nature of the overall goal that has been established. If the goal is oriented to fatal and injury target crashes, then these will be accumulated. If the goal is total target crashes, then these will be accumulated. The annual estimate of potentially treatable crashes or injuries can be extracted from the spreadsheet output in Step 3. For the routes chosen, divide the total crashes on each route by the number of years of data, and then sum across all chosen routes. 7. Repeat the above steps for each potential treatment type. As in Procedure 1 and 2A, the above steps are then re- peated for the second and subsequent potential treat- ment types. 8. Correct for multiple treatments on the same route. The user will again need to correct for multiple treat- ments on the same route. In general, the same rationale and procedure followed in Procedure 1 will be followed here. (See Step 8 of Procedure 1.) However, in this case corrections are made on a route basis rather than an intersection basis (i.e., either one treatment is specified for a given route and that route is removed from other treatment groups, or a correction in effectiveness is made for the second and subsequent treatment on the same route.) 9. Sum all expected crash injury reductions for all chosen treatment types and chosen target routes and compare that total to the established goal. 10. Add new treatments, new targets, or new approaches (e.g., inclusion of safety treatments in normal mainte- nance or rehabilitation efforts) until the goal is met. Again, the FHWA Sample Plan for intersections (25) provides additional discussion of this option. Procedure 3 – Choosing Intersection Treatments and Target Locations When Treatment Effectiveness in Terms of Crash/Injury Reduction Is Not Known As noted in the preceding section, the three procedures described above allow the user to choose intersection treat- ments and treatment targets for a given problem while en- suring that the economic value of the crash/injury reduc- tions will exceed the cost of implementing the program. All three procedures require that the treatments being examined each have a known level of effectiveness expressed in terms of an expected crash/injury reduction – a defined CRF or AMF. Unfortunately, many of the intersection treatments in the NCHRP Report 500 guide series do not have defined lev- els of effectiveness. Thus, economic analyses like those that are the basis for Procedures 1, 2A and 2B are not possible for these treatments. This Procedure 3 is aimed to help the user make an educated choice of which treatments will be most effective in their jurisdiction, and to help the user develop a targeting strategy for the treatment in cases where it is not to be applied jurisdiction-wide (e.g., where specific intersec- tions are to be targeted). In general, the choice between al- ternative intersection treatments will be based on the specific nature of the intersection crash problem, and the choice of target locations will be based on the determination of where the crash/injury problem of interest is found. A discussion of this more general procedure was included under the Proce- dure 3 subheading in Section III, and the reader should review that section. Procedure 3 is intended for application to tried or experi- mental treatments for which the analyst has decided that there is likely to be a crash/injury reduction benefit, but for which the analyst does not have sufficient evidence to estimate a spe- cific CRF or AMF value. In a case for which the analyst is able to estimate a specific CRF or AMF value, even if the estimate is only an approximation, it is recommended that Procedure 4 be used rather than Procedure 3. Data Needs The only required data for Procedure 3 are crash data that will allow the user to specify target crashes for each inter- section treatment under consideration (see Exhibit V-1 in the “crash data” description under Procedure 1). However, in order to target the treatment to specific intersections, the crash data must be “mileposted” or at least contain information on the county and route. The availability of intersection inventory data that can be linked to the crash data will improve both the treatment choice and the treat- ment targeting. 50

Procedure As described in Section III, Procedure 3 has two basic steps: • First, choose the best treatments (i.e., the intersection treatments most likely to be applicable in a given jurisdic- tion) from among the set of all intersection treatments pre- sented in the applicable NCHRP Report 500 guides. • Second, choose the routes or route segments to which the selected treatments should be applied. As described earlier in more detail, the choice of the best treatments from the listing of many potential intersection treatments can be based on the following factors: a) The many potential treatments judged to be the most effective, even given that effectiveness is unknown b) The relative magnitude of the crash types and severity levels that the treatment will affect c) The cost of the potential treatments per mile d) Other technical or policy considerations These factors must be combined in some fashion to decide which treatment to choose. While there are multiple ways of making this choice, the following represents one such proce- dure. 1. Prioritize the specific intersection problem(s) to be addressed. This is related to Factor b in the above list. Here, the issue is whether to treat angle, sideswipe, run-off-road, rear-end, head-on, or other intersection crash types, and at which intersection types (e.g., rural three-leg unsignal- ized intersections, urban four-leg signalized intersections, etc.). This prioritization will be based on the frequency and severity of the specific types of intersection crashes occurring in a user’s jurisdiction. Target crash types for each of the intersection treatments were defined under Procedure 1. For each crash type, the user could begin the process by analyzing 3 to 5 years of crash data to determine the frequency of each type. However, since some crash types are more severe than others (e.g., head-on crashes are more severe than sideswipe crashes), total crash frequency alone does not provide the complete answer. 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 (i.e., the total economic cost of crashes) within each target crash type. Information on economic cost per severity level within 22 different crash types including different types of run-off-road and head-on crashes can be found in Crash Cost Estimates by Maximum Police-Reported Injury Severity Within Selected Crash Geometries (22). This analysis of total crash cost will pro- vide the user with overall information on which intersec- tion crash type is most important. The user may further refine this analysis by examining crash frequency or total crash cost within intersection classes. If the crash data are mileposted and linkable in- ventory data are available, details of intersection types can be linked to each crash record (e.g., number of legs by type of traffic control). If inventory data are not available, there may be variables on the crash record itself which can be used in a less-detailed analysis (e.g., name of intersecting road, distance from some landmark). This analysis will then produce a listing of potentially treatable intersection crash types (perhaps by intersection class) that can be sorted by frequency or total crash cost, thus providing a ranked listing. For the higher-ranked crash types, the user can then conduct additional analyses to determine more of the specifics of the crash circum- stances (e.g., nighttime vs. daytime distributions of total crash cost). These additional “drill-down” analyses should be designed to provide additional information that could lead to the choice of one treatment over another (e.g., raised pavement markers are primarily effective at night or in rainy weather). 2. Identify possible treatments for use for each high-priority crash type. The user will then review the pertinent NCHRP Report 500 guides and list treatments that would be most appro- priate for each of the high-priority crash types identified in the above step. The choice should be limited to those treat- ment strategies that are classified as tried in the guides. (Proven treatments have known effectiveness levels and can be analyzed in one of the three procedures above.) 3. Rate the possible treatments based on estimated effectiveness. Since this procedure deals with treatment strategies with unknown effectiveness, this appears to be impossible. However, for a given set of possible treatments for a par- ticular crash type/intersection class combination, it may be possible to make a judgment concerning which treat- ment strategy would be expected to be most effective. For example, for rear-end crashes on high-speed roads, one would assume that left-turn lanes would be more effective than advance warning signs. At times, this will clearly be a very difficult judgment to make. 4. Choose the “best” treatment(s) by considering esti- mated effectiveness, cost per intersection and other technical and policy considerations. 51

The user will then combine the output of the steps above with at least two other factors in making a final de- cision on which treatment(s) to implement – the cost per intersection of the treatment and other technical and pol- icy considerations. Unfortunately, there are no good guidelines for how to “weight” the different factors. While problem size (total crash cost) and assumed treatment effectiveness are key factors, there may be technical, policy, and cost considerations that will remove certain treatments from consideration even if they are felt to be effective. The user will have to choose the final treatments based on “best judgment.” The procedure outlined above will at least ensure that the major factors in the decision are clearly defined. The output of this step will be one or more chosen treatments, with the nature of the treatment defining the specific crash types more likely to be affected (e.g., left-turn lanes will affect rear-end crashes). The user should be able to work backwards using the number of crashes likely to be affected by a given treatment and the cost of applying that treatment to a given population or location (see items b and c described at the beginning of this procedure) to determine the treatment effectiveness needed to maintain a B/C ratio greater than or equal to 1.0. Where: • B = economic benefit of applying a selected treat- ment to a given location or population • Ct = the cost of applying that treatment to the selected location or population B = N*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 reduc- tion 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. Solving for the treatment effectiveness, the equation reads: The analyst can then determine whether the calculated treatment effectiveness required to reach the breakeven point is likely to be achievable. 5. Target the chosen treatments to the intersections where the problem is found. Eff C N C t c = * B Ct ≥1 0. Since this procedure concerns treatment strategies with- out known effectiveness, it will not be possible to target the treatments based on any type of economic analysis such as those in Procedures 1, 2A and 2B. Instead, the treatment will be targeted to intersections showing the highest total crash cost or frequency, coupled with user judgment concerning the nature of the intersection and technical and political issues. If the crash data are mileposted, the user could (1) link crashes to routes and search for the locations of “clusters” of target crashes for possible treatment, or (2) use a network screening program similar to that described under Procedure 2A to identify 1-mile sections with the highest crash frequency or total crash cost. The windows identified by the network screening program could then be ranked by frequency or total crash cost to identify priority locations. The user would then correct for “treatment gaps” using the same logic provided in Procedure 2A. If the crashes are not mileposted, but there is informa- tion available on jurisdiction and route, the user could link crashes to routes within the jurisdiction and calculate the total crash cost or number of target crashes per mile by dividing the sum of the crash cost or the sum of target crashes on that route by route length. The user can then rank the potential routes for treatment based on this rate per mile and choose the routes to be treated based on the highest rankings plus other technical and policy factors. 6. Decide what to do with multiple treatments at the same intersections/routes. The above steps could possibly produce intersections or routes within a jurisdiction that could be treated with multiple treatments. Unlike the earlier procedures where it was possible to estimate combined effectiveness for multiple treatments on the same intersections or routes, since treatment effectiveness is not known here, the user will have to use other factors in the final treatment choice for these locations. If the potential treatment strategies still under consideration are characterized by different target crash types (e.g., rear-end crashes vs. sideswipe crashes), and if the crash data are mileposted or include route information, the user could use the outputs of Step 5 above in making the targeting decision. Step 5 produced total crash cost or crash frequency of each potential target section or route. For each intersection or route where multiple treatments are possible, the user could compare the crash frequency or total crash cost for each of the dif- ferent possible strategies. 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 the total crash cost or frequency for the other, the first would be a logical treatment choice. If the total crash cost or frequency for the different strategies is essentially the 52

same, the user will need to make the decision based on best judgment. 7. Add new treatments, new targets, or new approaches (e.g., inclusion of safety treatments in normal mainte- nance or rehabilitation efforts) until the available funding is used. In Procedures 1, 2A and 2B, an iterative process is used until sufficient treatment types and locations are selected such that the established crash reduction goal can be reached. In Procedure 3, without effectiveness measures for the treatments, it is not possible to verify whether or not a specific set of treatment types and treatments will meet the established goal. Therefore, the best that can be done is to proceed in selecting treatment types and treat- ments until the available budget for safety improvement has been fully committed. The total benefit of the selected program will not be forecastable, but the success of the program can be determined by evaluations conducted after its implementation. Procedure 4 – Choosing Treatments and Target Populations in Emphasis Areas for which Some Candidate Treatments Have Known Effectiveness Estimates and Other Treatments Do Not In many situations, users considering a safety improvement program in a particular emphasis area will need to consider both treatments that have known effectiveness measures and treatments that do not. In this situation, it is recommended that the user give priority to treatments that have known effective- ness measures (proven treatments). Treatments that have been used extensively but for which effectiveness measures are not available (tried treatments) should then be considered. Experimental treatments may have a modest role in a safety improvement program, particularly if the program is structured to evaluate the effectiveness of the experimental procedure. The recommended planning approach in this situation is a hybrid of Procedures 1, 2A, 2B, and 3 described above. Procedure 1. Determine if proven treatments can meet the established goal. Consider treatments with known effectiveness meas- ures (proven treatments) using either Procedure 1, 2A, or 2B as appropriate, depending on the types of data available. Determine the crash/injury reduction achieved and compare it to the established crash/injury reduction goal. If the goal has not yet been met, proceed to Step 2. 2. Consider tried treatments to supplement the proven ones. This step would involve consideration of treatments without known effectiveness that have been used exten- sively by highway or driver/vehicle agencies (tried treat- ments). If it is possible to estimate the effectiveness of these treatments based on imperfect information, then proceed to Step 3; otherwise, proceed to Step 4. 3. Estimate the effectiveness of tried treatments if possible, and analyze them using the appropriate procedure above. This step involves attempting to estimate the effective- ness of treatments without known CRFs or AMFs. Note that estimating treatment effectiveness is very difficult and can lead to poor treatment choices unless the estimates are re- alistic. This estimation was not suggested in Procedure 3 for this reason. It is only suggested at this point since the user has already considered all proven treatments before reaching this stage. It is suggested that the following guide- lines be used in making such estimates: a) In general, be as conservative as possible. Very few treat- ments can be expected to affect crash frequency by more than 15–25 percent. b)When possible, formulate an effectiveness estimate that is applicable to particular target crash types only, not to total crashes. c) Base estimates for tried treatments on CRFs for similar treatments if they exist. Once effectiveness is estimated, apply Procedure 1, 2A or 2B as appropriate, depending on the types of data avail- able. Determine the crash/injury reduction achieved in Steps 1 and 3 combined and compare it to the established crash/injury reduction goal. If the goal has not yet been met, proceed to Step 4. 4. Consider additional tried treatments. For treatments for which reliable effectiveness measures cannot be estimated in Step 3, apply Procedure 3 to select additional treatment types and target locations until all available funds have been budgeted. 53

Next: Section VI - Special Road User Populations »
Safety Data and Analysis in Developing Emphasis Area Plans Get This Book
×
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

TRB's National Cooperative Highway Research Program (NCHRP) Report 500, Vol. 21: Guidance for Implementation of the AASHTO Strategic Highway Safety Plan: Safety Data and Analysis in Developing Emphasis Area Plans provides guidance on data sources and analysis techniques that may be employed to assist agencies in allocating safety funds.

In 1998, the American Association of State Highway and Transportation Officials (AASHTO) approved its Strategic Highway Safety Plan, which was developed by the AASHTO Standing Committee for Highway Traffic Safety with the assistance of the Federal Highway Administration, the National Highway Traffic Safety Administration, and the Transportation Research Board Committee on Transportation Safety Management. The plan includes strategies in 22 key emphasis areas that affect highway safety. The plan's goal is to reduce the annual number of highway deaths by 5,000 to 7,000. Each of the 22 emphasis areas includes strategies and an outline of what is needed to implement each strategy.

Over the next few years the National Cooperative Highway Research Program (NCHRP) will be developing a series of guides, several of which are already available, to assist state and local agencies in reducing injuries and fatalities in targeted areas. The guides correspond to the emphasis areas outlined in the AASHTO Strategic Highway Safety Plan. Each guide includes a brief introduction, a general description of the problem, the strategies/countermeasures to address the problem, and a model implementation process.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!