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In this scenario, the Roadways Manager will be studying road safety at the two-way stop-controlled intersections in town, and identifying if anything should be implemented to address safety concerns. As such, this scenario will conduct all of the steps in Figure 1.
The first step in studying the two-way stop-controlled intersections is compiling and evaluating the available data. The data available influences the type of analyses that can be conducted. Typically, the information available can be divided into anecdotal information and quantitative information.
Is Anecdotal Data Valuable?
Anecdotal data may provide information that is not shown in the crash reports. This information can be valuable in providing clues on where to start a data-driven investigation. The challenge with anecdotal data is sorting out what issues are the perceptions of safety issues versus actual addressable safety issues.
In this scenario, the anecdotal information is gathered from the town commissioners and concerned residents. For many years, residents and commissioners alike have expressed concerns about vehicles making left turns onto and off of the main roads at intersections in town. While different intersections have different issues, in summary residents have expressed concerns about:
These concerns can be a good source of information and provide useful clues as to what should be studied at sites; however, this type of information should be supported with quantitative data when possible to separate perception from fact.
Table 1 shows the types of quantitative data that can be used for this type of safety analysis, and shows the data types that this scenario assumes are available. More information about sources for and how to work with each data type follows in this section.
The crash summary data and printed crash records can be acquired from the local law enforcement agency or from the state department managing the crash data.
Local law enforcement agencies typically only hold crash records created by their officers, so crash records may be missing if there are other police agencies that have jurisdiction in the area. Only reported crashes will be in the official records. Also, most states require crashes to be reported when the dollar value of damages exceeds a minimum threshold or if there is an injury. Crashes that result in limited vehicle property damage may not be reported to police. Usually, the more severe the crash, the more likely the crash gets reported to the police.
Depending on what data sources are available, compiling data may be a step that is completed before or with the network screening activity. Agencies that have data in electronic formats, such as GIS databases, can compile and report on data with little effort. As such, compiling crash summaries for every intersection in the road network is quick and easy; however, agencies that have to manually compile data may only create summary tables for locations identified in the screening step due to the effort involved.
What is a GIS Layer?
GIS can be thought of as a map combined with a database. All the information contained in the database is related to a physical location contained in a map. GIS users can apply a legend or a theme to present specific data on the map as a data layer. Various layers of data items can be overlaid to visualize relationships.
Examples: A road layer can be overlaid with a land use layer, or a bridge location layer (or both), and then crash locations can be also added.
In this example, the Roadway Manager and staff have been maintaining a crash data spreadsheet for the town’s intersections for many years. Each year, they acquire the intersection crash data from local law enforcement, and record each intersection’s crash data in a spreadsheet. The type of information they record for each crash includes:
Crash Severity KABCO System
The KABCO Scale to classify crashes by injury severity. The letters represent injury levels:
The severity of a crash is based on the greatest level of severity of injury occurring in the crash. For example: If someone is killed in a crash, the crash is labeled as a “K” or fatal crash.
Crash data can be summarized from individual crash records and then all of the crash records for a given location can be summarized. Table 2 shows an example of summary details from individual crash records at one of the intersections – First Street and Main Street. Table 3 shows how all crash data for all of the locations can be summarized.
Although crash record forms vary from state to state, Table 2 shows an example of the common information that might be compiled from a crash record form. This information can provide clues to crash contributing factors in later diagnosis steps in the process.
In this scenario, a mapping tool is available; therefore, the crash data also can be summarized as demonstrated in Figure3. This map shows total number of crashes at the two-way stop-controlled intersections in this analysis. The size of the dot is scaled to the total number of crashes at each intersection for the study period. However, as discussed in the Toolkit, a map like this could alternatively show the summary of crashes by type or severity at the study intersections, or distribution of crashes by year.
Figure 3. Map of Intersection Crashes – 2010 to 2012
The availability of major and minor street average daily traffic (ADT) volume data allows the practitioner to conduct network screening analyses using crash rate or critical crash rate methods. Chapter 4 of the Highway Safety Manual presents these methods. However, Annual Average Daily Traffic (AADT) volumes are not always available because of the expense of a regular traffic counting program. For most studies, ADT collected at the study sites on any day is used to estimate AADT.
Consideration When Applying Crash Rate Network Screening Method
Crash rate network screening is useful because:
However, if multiple sites have the same number of crashes, the lower volume locations will always show higher crash rates. So, crash rates should only be used to compare sites with traffic volumes in the same range.
Often, the ADT at intersections is converted to Total Entering Volume (TEV). The TEV is the sum of all traffic entering an intersection. TEV of the study intersections can be mapped using a GIS database or can be summarized in a table.
For this network screening scenario traffic volume data will not be used.
As described in the Toolkit, roadway characteristics data may be available in-house from the engineering team, or externally from the state DOT roadway databases or as-built drawings, on-line mapping tools such as Google Street View™ mapping service and/or site field visits. To the greatest extent possible, a site visit should always be part of the safety analysis process.
The types of roadway characteristics that can be collected in the field site visit are:
More information about the type of roadway characteristics data to collect is included in Step 1 of the Toolkit.
Intersection characteristics information will be used in this scenario as part of the site investigation and diagnosis. The type of information that will be used includes:
Table 4 demonstrates how this type of information might be summarized in a table and what this data set might look like for a subset of intersections from Table 3.
Table 4 includes information about the number of driveways within 250 feet of the intersection. Depending on traffic volume and pedestrian and bicycle traffic, often as the number of driveways increases, rear-end crashes will increase with the amount of activity along and across the roadway. So, the number of driveways can be useful in understanding crash contributing factors near intersections. This information is not often collected and maintained in a database, but it can be collected using recent aerial photos, an on-line tool like Google Street View™ mapping service or during the field visits.
Finally, there may be other agency-specific or statewide documents that contain information useful for the analysis. For example, the state SHSP may contain information about the characteristics of intersection crashes in state and preferred actions to address these strategies. Additionally, if the state is an intersection focus state, the state DOT may have an intersection plan that can be useful in providing information on identifying and addressing intersection safety issues.
The state’s SHSP has identified that intersection crashes are a focus area. As such, there are strategies and actions available to assist with addressing intersection crashes. The following are some of these strategies and actions:
The state also recently provided Highway Safety Manual training that included information about network screening. This information was drawn largely from the FHWA Highway Safety Manual training available on-line.
When applying any network screening program, it is often useful to apply more than one network screening method to compare and contrast the results and draw yet more conclusions from the findings. Based on the data and resources available, this example scenario applied frequency-based and equivalent property damage-only (EPDO) network screening methods. Step 2 of the Toolkit provides more information about this and other network screening methods.
Threshold of Performance
A challenge with frequency-based network screening is that there are no specific indications as to whether the observed crash frequencies should be considered high, typical, or low for the particular study intersection group. There is no threshold or performance measure that says if the frequency is greater than x, the site has potential for safety improvement.
Crash frequency is one of the most basic network screening methods. In this method, the intersections are grouped, evaluated, and compared according to categories of traffic control; number of approach legs; and traffic volume range (e.g., higher-volume intersections separated from lower-volume intersections). For example, a network screening program could consider all three-legged signalized intersections in one group, and all four-legged two-way stop-controlled intersections in another group (as in this scenario).
The crash frequency measure is the total number of crashes over the analysis period or average number of crashes per year for the study intersections. The intersections are ranked from highest to lowest total crash frequency over the analysis period or average crash frequency per year. The crash frequency measure can be applied to total crashes, fatal and serious injury only crash severities, or by a particular crash type.
Crash Rate Analysis
If traffic volumes at the intersections were available, a crash rate analysis could also be conducted. Crash rate (crashes divided by traffic volume) could be conducted on total crashes, fatal and serious injury only crashes, or a particular crash type.
If using crash rates, be sure to subdivide the sites into categories with comparable traffic volumes. For example:
Table 5 illustrates how intersections could be ranked according to crash frequency. The sites are ordered by total crash frequency; the severity data is provided for reference.
The equivalent property damage-only (EPDO) method is documented in the Highway Safety Manual. In this method, weighting factors related to the societal costs of fatal, injury, and property damage-only crashes are assigned to crashes by severity to develop an equivalent property damage-only score that considers frequency and severity of crashes. The sites are ranked from high to low EPDO score. Those sites at the upper end of the list can be selected for investigation.
The major steps in this method are:
| Severity | Comprehensive Crash Cost (AASHTO, Highway Safety Manual 2010, Chapter 7) |
|---|---|
| Fatal | $4,008,900 |
| Injury A, B, and C | $82,600 |
| Property Damage Only | $7,400 |
The crash severity weighting factors are calculated as a function of the PDO crash cost. The fatality EPDO weighting factor is:
The injury EPDO weighting factor is:
The property damage only (PDO) weighting factor is:
For example, the EPDO score at Intersection A is:
Table 6 shows the EPDO score for each intersection. Ranked by EPDO, the top three intersections are Intersections L, J, and O; however, ranked by frequency the top three intersections are C, L, and E (see again Table 5).
Systemic Analysis is another approach for network screening that is useful in the rural local and Tribal context. In the scenario presented here, the practitioner is evaluating safety conditions at spot-specific locations. The systemic approach analyzes crash history on an aggregate basis to identify:
A systemic safety approach works by identifying which roadway characteristics (such as road width, shoulder width, posted speed, intersection control, urban or rural environment, or number of intersection approaches) are included at a large proportion of the road network’s crash sites. Once these problematic roadway characteristics are known, locations with these characteristics can be identified, and countermeasures targeting them implemented so that crash risks are reduced across the road network.
The major steps in the systemic process are:
The FHWA Systemic Safety Project Selection Tool, published in 2013, thoroughly describes these steps. See the resources section for more information.
As an outcome of a systemic analysis the agency will be able to understand roadway and roadside features that show higher risk than other features, have a list of treatment types that can potentially address risk, and have a list of sites for potential treatment.
For example, Figure 4 is taken from the FHWA Systemic Safety Project Selection Tool. In this case study, Thurston County Public Works in Washington State compared the proportion of severe curve-related roadway departure crashes on various functional classifications of roadways to the proportion those functional classifications represent of the entire County roadway system. The data show that the focus crash type – roadway departure crashes – occurs on roadways with a Rural Major Collector functional classification in a greater proportion than this roadway type represents for the County system. Based on this descriptive statistics analysis, Thurston County chose Rural Major Collector functional classification as a risk factor.
Figure 4. Evaluation of Roadway Functional Class as a Potential Risk Factor
Source: FHWA.
At the conclusion of this step, it would be useful to start the process of documenting the project analysis. An example document outline through this stage of the analysis is shown below.
Introduction
Data Collection and Evaluation
Network Screening Results
In this step, a subset of the sites evaluated in the network screening process is selected for detailed investigation. Step 3 of the Toolkit provides more information about the process and reasons for selecting or not selecting sites for more detailed investigation.
Sites can be eliminated from the remainder of the analysis for a variety of reasons, including:
Alternatively, sites can be selected for analysis if future development or construction is anticipated that could substantively change conditions. In this case, the site could be investigated for enhancements to implement with other upcoming changes.
Table 7 shows crash severity, total crash frequency, and the EPDO score for each intersection. The table also shows which intersections were selected for more detailed investigation and why. As shown, of the 15 sites included in the stop-controlled intersection safety plan, three have been selected for more detailed site investigation:
Note that even though Intersection J has a high EPDO score, high crash frequency, and one of the fatal crashes, it was not selected for investigation because there already are plans to signalize the intersection – this should address the issues at this location.
Intersection B was not selected for analysis either. It has a relatively high EPDO, and one of the fatalities occurred here. However, the issues at the location are known and have been previously studied.
In the next phase of the analysis, each of the selected sites will be studied in more detail to understand crash characteristics, potential contributing factors, and to identify possible countermeasures.
At the conclusion of this step, the practitioner should continue documenting the analysis results and the reasons for selecting sites for detailed investigation. At this stage, there is an additional step to the document outline – Site Selection for Detailed Investigation.
Introduction
Data Collection and Evaluation
Network Screening Results
Site Selection for Detailed Investigation
In this step, the sites identified for further evaluation are studied in more detail. The purpose of the detailed analysis at this stage is to identify potential crash contributing factors and identify potential treatments to address the identified safety concerns. The diagnosis steps and methods for identifying countermeasures are the same for each site under investigation. The issues and results of the analysis differ by site; however, the process for conducting this analysis does not change. Therefore, in this example, the process of diagnosis and selecting countermeasures is shown for one intersection only: Intersection L – First Street and Main Street. In practice, this process would be repeated for each site selected for detailed analysis identifying specific solutions for each site.
The Toolkit provides many different resources and tools for site diagnosis and countermeasures selection. A field visit is a very important step in conducting a site diagnosis. As described in the Toolkit, the field visit should be conducted in daylight and, if possible, under dark conditions. The field visit also could be conducted in the context of a Road Safety Audit (RSA), as described in the Toolkit.
Field Visit
It is important for the analyst to make a field visit if possible. They should view the site first hand and observe traffic and other features that data or photos alone cannot convey.
The type of data collected during a site visit should include number of lanes, lane width, shoulder width and shoulder type, sight distance where it appears limited, posted speed limit, and other signage. Videotape or photographs are also very helpful in documenting conditions.
A nighttime field visit is very important to evaluate the signing and pavement marking in dark conditions. The RSA Guidebook and Highway Safety Manuals have prompt lists that can assist in field visit data collection. See the Toolkit for more information on these resources.
Three tools that are integrated into field visits and RSAs and are useful in identifying potential crash contributing factors and countermeasures are collision diagrams, condition diagrams, and the Haddon Matrix. These tools are described below.
In this scenario, collision diagram, site condition diagram, and Haddon Matrix are developed for Intersection L (Main Street and First Street). These tools can be similarly applied for all intersections selected for detailed site investigation.
Table 8 shows the crash type summary table for Intersection L. Figure 5 shows the collision diagram for Intersection L.
Figure 5. Collision Diagram for Intersection L (First Street and Main Street) – 2010 to 2012
Reviewing the table and the diagram shows:
Figure 6 shows the site condition diagram. As shown, during the field review, staff noted that the intersection approaches have thick vegetation on the northwest and southeast corners of the intersections and there is no illumination on all four intersection corners. Furthermore, the intersection is at the crest of the vertical curve along the Main Street alignment. There also are two driveways off First Street – a residential driveway off westbound First Street located 50 feet east of the intersection, and a school driveway off eastbound First Street located 100 feet west of the intersection.
Figure 6. Condition Diagram for Intersection L (First Street and Main Street)
Table 9 shows the Haddon Matrix developed from the study intersection’s crash records. As described in the Toolkit, the Haddon Matrix provides a framework for reviewing crash records and understanding potential contributing factors in the context of the time of events: before the crash, during the crash, and after the crash. The time of events is divided into the three major categories of contributing factors: the human, the vehicle, or the environment. For the most part, information and clues for the type of engineering solutions that might be useful can be drawn from the physical environment column.
The field visit, crash data summary, and evaluation, collision diagram, condition diagram, and Haddon Matrix show:
Based on this summary, the left turn crashes from eastbound First Street onto Main Street and northbound Main Street onto First Street are the most critical and concerning; therefore, safety treatments will focus on reducing frequency and severity of these angle crashes.
The resources available to identify countermeasures are summarized in Step 4 of the Toolkit. These resources are the same whether one site or many sites are being investigated: the web-based CMF Clearinghouse, Part D of the Highway Safety Manual, the NCHRP 500 Reports, and the FHWA Office of Safety Proven Countermeasures web page. Step 4 of the Toolkit provides detailed information about each of these resources. This scenario uses the CMF Clearinghouse and the Highway Safety Manual as resources for identifying possible countermeasures to address angle crashes at the intersection.
Summary of Crash Modification Factors (CMF)
A CMF is a multiplicative factor used to compute the expected number of crashes after implementing a given countermeasure at a specific site. The CMF is multiplied by the expected crash frequency without treatment to show the expected crash frequency with the treatment.
A CMF greater than 1.0 indicates an expected increase in crashes, while a value less than 1.0 indicates an expected reduction in crashes after implementation of a given countermeasure. For example, a CMF of 0.8 indicates an expected safety benefit; specifically, a 20 percent expected reduction in crashes. A CMF of 1.2 indicates an expected degradation in safety; specifically, a 20 percent expected increase in crashes.
CMFs for several countermeasures can be multiplied to reflect the application of multiple safety countermeasures at the same location.
Table 10 shows countermeasures identified as possible treatments to address the scenario’s turning movement and nighttime crashes. The potential countermeasures address sight distance constraints, lighting concerns, and angle crashes. The table also shows the CMF values and construction costs estimates for the countermeasures (construction costs for these countermeasures are based on 2013 average unit price for Michigan Department of Transportation).
Vegetation Control. Through vegetation control, maintenance staff can remove plants, clear tree branches, and trim brushes that block a driver’s line of sight at an intersection. Drivers approaching an intersection need a clear line of sight to the intersection and along the crossing roadway to perceive and react to potential conflicting vehicles, bicyclists, and/or pedestrians. The sight lines required for drivers to see approaching traffic form a sight triangle over an intersection corner, as shown in Figure 7.
Intersection Illumination. Intersection illumination can provide lighting for drivers, pedestrians, bicyclists, and other road users to see each other approaching the intersection. Streetlights also can illuminate intersection characteristics for users such as signage, striping, and shoulder. In rural situations, intersection illumination can act as a point of reference or landmark for drivers so they can see the intersection that they are approaching well in advance of their arrival. This helps in navigation.
Signalization. A traffic signal warrant is an analysis used to determine whether conditions warrant traffic signal installation at an intersection. If traffic signal warrants are met, converting an intersection from stop-controlled to signal-controlled provides specific signal phases for turning and through vehicles at the intersection, thereby reducing the number of possible collisions. MUTCD should be used to conduct the traffic signal warrant analysis.
Figure 7. Intersection Sight Distance Triangle
Source: Vegetation Control for Safety, A Guide for Local Highway and Street Maintenance Personnel, FHWA Office of Safety.
When choosing a CMF for a particular countermeasure, it is common to have to select one value among many. To select a CMF, the practitioner should consider:
For example, as shown in Figure 8 the CMF Clearinghouse provides six different CMFs for the countermeasure “Increase Triangle Sight Distance.” Differences in these CMFs include:
The practitioner will have to review the Clearinghouse’s detailed information to select conditions most applicable to the site under investigation. In this example, Table 10 also shows the three sight distance triangle CMFs with the highest star rating and most applicable to our site conditions.
Figure 8. Increase Sight Distance Triangle From CMF Clearinghouse
Source: CMF Clearinghouse.
At the conclusion of this step, the agency will have identified potential countermeasures for each site selected for investigation in Step 3. At this stage, the project documentation could be updated to include the diagnosis results and selection of potential countermeasures. An example of the documentation outline is shown below.
Introduction
Data Collection and Evaluation
Network Screening Results
Site Selection for Detailed Investigation
Diagnose Results and Selection of Countermeasures
The purpose of the countermeasure evaluation and prioritization step is to review the potential countermeasures and select the most feasible countermeasure for the site under investigation. The process for prioritizing and selecting countermeasures among a number of optional countermeasures for one site can range from a quantitative benefit/cost analysis to a qualitative rating process using high, medium, and low (or good, fair, poor ratings), or a hybrid of both.
The criteria that influence the quantitative or qualitative analysis will vary from agency to agency. Some typical criteria are:
Step 5 of the Toolkit and User Guide #1 provide an example of a countermeasure selection process applying a qualitative ranking process.
This User Guide uses a quantitative benefit/cost analysis to identify the most feasible treatment at the site. Conceptually, in a benefit/cost analysis, the benefits and costs of implementing a countermeasure are converted to a present-year dollar value. The total present dollar value of the benefits is divided by the total present dollar value of the costs. If the ratio exceeds 1.0, the treatment is considered economically feasible. The higher the benefit/cost ratio, the greater the value of the project under consideration. Step 5 of the Toolkit presents more information about benefit/cost analysis.
The calculation steps for benefit/cost ratio are the same for all countermeasures; although actual benefits and costs of each countermeasure vary. As such the benefit/cost ratio calculations for only one of the three possible countermeasures identified for Intersection L – Main Street and First Street – vegetation control is shown. Table 11 summarizes the results of the all three B/C calculations.
The major steps in this benefit/cost analysis are:
| Vegetation Control CMF by Severity | CMF |
|---|---|
| Fatal Crash | 0.44 |
| Injury Crash | 0.53 |
| PDO Crash | 0.89 |
If there were crashes with other severity types, the potential savings per year would be calculated in a similar fashion: the dollar value of the crash severity type multiplied by the potential savings in crash severity type per year.
Societal Cost
There are costs to society of a death or injury occurring in a crash. Costs include (defined by National Safety Council): wage and productivity losses, medical expenses, administrative expenses, motor vehicle damage, and employers’ uninsured costs.
This step calculates the net present value factor for calculating the potential benefits in every year. The equation for net present value factor is:
i = Minimum attractive rate of return or discount rate (this scenario assumes a return rate of 4 percent or i = 0.04)
y = Year in the service life of the countermeasure, one calculation for each year of life.
For example, in Year 10, the present value factor is:
Table 11 shows the present value of the benefits and costs for each year of effectiveness of the vegetation control. As shown, saving $38,822 (today’s dollars) in injury crashes per year in Year 10 has a present value of $314,881.
This scenario assumes it costs $4,500 per acre to clear sight triangles plus $500 per year to maintain conditions. This scenario further assumes that every third year, the more expensive sight distance maintenance will be required. These costs are for this example only and should not be applied to any jurisdiction-specific analyses. In this scenario it is necessary to clear one-half acre of vegetation from the northwest corner of the intersection.
Table 11 also shows the net present value of the costs to maintain sight distance at the intersection for 20 years. As shown, the present value of spending $2,750 in Year 10 is $22,305; and the total net present value for all 20 years to maintain sight distance is $200,548.
In practice, Steps 1 through 4 would be conducted for each countermeasure being considered for implementation.
Table 12 shows summary-level results of this calculation for each countermeasure. The benefit/cost ratio is the net present value of the benefits divided by the net present value of the costs. As shown, the benefit/cost ratio (B/C) for vegetation control is:
The results of this analysis show that implementing vegetation control has the highest benefit/cost analysis, assuming the sight distance clearance is maintained at all times for 20 years. Therefore, this treatment is selected to be implemented and maintained at the intersection.
There also is a relatively high benefit/cost ratio associated with signalizing the intersection. If traffic signal warrants are met at the intersection and initial construction costs are available, this also could be considered a beneficial modification to the intersection.
At the end of this stage, it is appropriate to expand the project documentation again to include an explanation of the analysis, prioritization, and recommended treatments. This information also could be presented to agency leadership for review, input and approval if necessary. At this stage, the project documentation outline could be:
Introduction
Data Collection and Evaluation
Network Screening Results
Site Selection for Detailed Investigation
Diagnose Results and Selection of Countermeasures
Countermeasure Prioritization
Recommendations
At this stage of the process, diagnosis, countermeasure selection, and prioritization would be complete for all intersections selected for additional analysis in Step 3. Next, the agency works on implementing the countermeasures.
Obtaining the necessary human and financial resources is a major consideration in implementing any safety project or program. Harnessing local funding sources and staff resources is often the quickest way to implement projects. For example, maintenance or public works staff can implement low-cost projects such as maintenance or replacement of signs, maintenance of striping, and/or vegetation control as part of their regular duties.
In addition to local funds, the Local Technical Assistance Program (LTAP) web site describes the various types of local agency support provided by state DOTs – a useful first stop for identifying the resources available by state. The Toolkit also provides more information about funding opportunities.
The FHWA’s Vegetation Control for Safety Guide provides guidance on checking and improving intersection sight distance. Drivers approaching an intersection need a clear line of sight to the intersection and along the intersection to react to potential conflicting vehicles, bicyclists and pedestrians to avoid a collision. Figure 9 from the FHWA’s guide shows the sight triangles formed by the sightlines of two vehicles approaching an intersection.
Figure 9. FHWA’s Vegetation Control for Safety Guide – Sight Triangle Diagram
Source: Vegetation Control for Safety, A Guide for Local Highway and Street Maintenance Personnel, FHWA Office of Safety.
In this scenario, vehicles stopped at the intersection on First Street need sight distance to see vehicles approaching from either direction of Main Street, so that the stopped driver can safely turn left, turn right, or proceed across the intersection. Of the three options, the sight distance needed for a left turn is the longest and can be used to establish the corner sight triangle for both legs of the minor road.
Vegetation at the northwest corner of the intersection is cleared and maintained for 20 years. No shrubs or plants within the corner sight triangle should be allowed to grow more than three feet high. Trees over three feet high are removed. In cases where sight distance is limited by vegetation on private property, staff can work with the private property owner to identify opportunities for controlling and managing vegetation.
Finally, if other aspects of the project have been documented, it would be useful to add the results of this step as well. At this stage, the project documentation outline could be:
Introduction
Data Collection and Evaluation
Network Screening Results
Site Selection for Detailed Investigation
Diagnose Results and Selection of Countermeasures
Countermeasure Prioritization
Recommendations
Final Comments
Three to five years after implementing and maintaining the treatment, agency staff should measure the effectiveness of the treatment. This can be completed by tabulating the number and type of crashes at the site since implementing the improvement – essentially repeating Step 1 and recreating Table 3 in this scenario and/or conducting more rigorous before-and-after study evaluations. Step 7 of the Toolkit provides more information about optional methods for evaluation.
Use Caution with Simple Before-and-After Analysis Results
A simple before-and-after analysis does not provide a quantitative result. Because of statistical limitations of crash data, a simple analysis is not sufficient to make an accurate quantitative assessment.
In this scenario, assuming traffic volume does not change dramatically, crash records for the three-year period after implementing the treatment are collected and compared to the three-year period before treatment implementation.
Table 13 shows how the before-and-after crash data can be tabulated. In this scenario, three years after clearing and maintaining the sight distance triangles, angle crashes at the intersection decreased from eight in the three-year period before the treatment to two in the three-year period after, or a 75 percent reduction in angle crashes. Because of statistical issues associated with crash data (explained in the Toolkit) – it should not be concluded that there was a 75 percent reduction in angle crashes. It can be concluded that angle crashes have decreased, but this analysis is not statistically rigorous enough to quantify the change in crash frequency.
Table 13 also shows that there has been a slight increase in rear-end crashes from the before to the after period. This could be due to the natural variation in crash frequency or it could reflect a new crash pattern emerging. It would be valuable to continue monitoring the intersection to ensure there are no additional modifications necessary.
It also is important to note that if traffic volume changes substantially after implementing the treatment at the site, this type of simple before-and-after crash analysis will be less valid because the change in traffic volume may be influencing the change in crash frequency or severity.
Documenting the results of the effectiveness analysis in a memo to file or for presentation to governing board would be useful. This could demonstrate the value of the project in the specific jurisdiction and serve as a resource if similar projects are being considered in the future.
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