Intersection Safety Implementation Plan Process

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Step 5: Develop a Straw Man Outline

Once the countermeasures considered acceptable by the State to implement are identified (Step 3) and the data analysis is complete (Step 4), the State can develop a straw man outline to achieve the intersection crash reduction goal (Step 1). An example data analysis package and straw man outline can be found on the FHWA Intersection Safety web page http://safety.fhwa.dot.gov/intersection/).

Establish Threshold Crash Levels

The first step in creating the straw man outline is establishing threshold crash levels for the intersection and crash types that each countermeasure is intended to impact. At the very least, separate thresholds should be calculated based on road ownership and location (i.e., State/local, rural/urban) such that each crash type may have up to four thresholds depending on where the crashes occur. Crash threshold levels can be established based on a number of factors including:

  • The level at which a countermeasure is cost-effective (B/C ratio usually set at 2.0 or greater).
  • The number of improvements, the unit cost of the countermeasure and the availability of funds per threshold level.
  • The lowest number of crashes per intersection considering exposure levels that may reasonably indicate a recurrent crash problem that can be mitigated by the countermeasure (usually not less than three crashes per intersection in 5 years at very low-volume intersections).

Improvements deployed on a systematic basis have to be cost-effective, and a B/C analysis is used to make the determination. The conventional analysis uses the following formula to compute the B/C:

Equation. B/C equals the result of Number of Crashes times CRF times Average Crash Cost all divided by Annual Cost.

Unlike a conventional analysis, the B/C is given or set. The answer one seeks is the threshold, the minimum number of targeted crashes per intersection needed to make the countermeasure cost-effective. The threshold is represented by the number of crashes in the conventional B/C formula above.

The formula used to establish the threshold is as follows:

Equation. T equals the result of Annual Cost times B/C divided by the result of CRF times Average Crash Cost.

Where:

  • T = Threshold – Minimum number of targeted crashes per intersection needed to make the countermeasure cost-effective.
  • Annual Cost = Annual cost of the improvement.
    • If the improvement involves a construction project, annual cost is the construction cost averaged over the expected life of the project.
    • If the improvement is an education or enforcement initiative, annual cost is the annual cost of a full year of enforcement and education.
  • B/C = A set B/C ratio used to determine the threshold number of intersection crashes. In this case, a B/C value of 2.0 may be used.
  • CRF = Estimated crash reduction factor, or effectiveness, of the strategy to reduce targeted crashes. It is expressed in terms of the percent of targeted crashes reduced.
  • Average Crash Cost = Average cost of targeted crashes using the cost data in Table 4 and the number of injury types for the targeted crashes.

As an example, consider a signal update for State urban intersections where:

  • Annual Cost = $3,000 ($30,000 averaged over 10 years).
  • B/C = 2.0.
  • CRF = 0.30.
  • Average Crash Cost = $40,000 (estimated from the distribution of fatalities, injuries, and property damage crashes for State, urban, signalized intersections).

Equation. T equals the result of 3,000 times 2.0 divided by the result of 0.30 times 40,000, which equals 0.50.

This example shows that the threshold should be 0.50 crashes annually, or between 2 and 3 crashes in 5 years. The results indicate that it does not take many crashes to apply low-cost countermeasures cost-effectively. However, when considering scarce resources, two additional factors need to be considered:

  • Establishing a very low crash threshold (e.g., less than 5 crashes in 5 years) may increase the randomness of crash occurrence and not reflect intersections with persistent and repetitive crash occurrences. It is suggested that a minimum of five targeted crashes in 5 years be the base threshold used in the analyses for most intersections. Intersections with four or even possibly three crashes in 5 years may be selectively added to the list if the traffic volumes (exposure) are extremely low.
  • Considering the relationship of crashes per intersection, number of intersections, and number of statewide crashes, as higher thresholds are set (crashes per intersection), the B/C will be larger and the total costs will be smaller, but the overall crash reduction also will be smaller.

As a result, scarce resources and achieving the intersection crash reduction goal have to be balanced.

Table 27 shows a sample distribution of crashes at State, urban, signalized intersections. If a State starts at the base level to achieve a B/C of 2.0 or greater (i.e., 3 or more crashes per intersection), then 2,955 intersections would be improved. This would encompass 97.5 percent of all signalized intersection crashes. However at $30,000 per intersection improvement, the costs to improve all 2,955 intersections would be close to $90 million. If the threshold is increased to 5 crashes per intersection, 2,487 intersections would be improved, encompassing 95.3 percent of all State urban signalized crashes. The costs for this level of improvement would be close to $75 million. Assuming funds are not that plentiful, if a threshold level of 50 crashes per intersection is selected, only 371 intersections would need to be improved at a cost of about $10 million. This would still encompass over 38 percent of all State, urban, signalized crashes.

Table 27: Sample Distribution of Crashes at State Urban Signalized Intersections
Number of Crashes per Intersection Number of Intersections Cumulative Intersections Cumulative Percent Cumulative Crashes Cumulative Percent
100 and greater 63 63 1.44 8,058 10.90
50 – 99 308 371 8.46 28,570 38.65
30 – 49 461 832 18.98 46,320 62.67
20 – 29 483 1,315 30.00 57,986 78.45
10 – 19 633 1,948 44.43 66,850 90.44
5 – 9 539 2,487 56.73 70,471 95.34
4 206 2,693 61.43 71,295 96.46
3 262 2,955 67.40 72,081 97.52
2 403 3,358 76.60 72,887 98.61
1 1,026 4,384 100.00 73,913 100.00
Total 4,384 4,384 100.00 73,913 100.00


Develop Detailed Straw Man Tables for Each Countermeasure

The straw man consists of a set of countermeasures, deployment levels, costs, and safety impacts (usually defined in terms of the overall goal (i.e. annual lives saved)), which collectively can achieve the overall intersection safety goal. Each countermeasure needs to be investigated in terms of its deployment levels, costs, safety impacts, and relative contribution in achieving the overall intersection safety goal using data from the data analysis. An example of the tabulation of intersections that can be considered for the basic set of sign and marking improvements for State stop-controlled intersections is shown in Table 28.1 In this table, 1,221 intersections had 6 or more crashes within the crash history period. A 6-year crash history period was used in this evaluation because it was available and provided more stable data than 5 years of data. In addition, no significant changes in traffic or roadway features occurred during the 6 years.

Not all of the intersections shown in Table 28 will end up as improvements. Some will have been upgraded previously; field reviews of others will show that sign and marking enhancements do not make sense. In this case, an assumption is made that only 80 percent of the intersections will remain candidates for the sign and marking enhancements after the field review. Each State can establish its own estimated retention rate for the improvement estimate. If time is available, a more accurate means of developing the estimate is to field review a random sample of the candidate intersections beforehand and use the percentage of these intersections in which enhancements are likely for the estimate.

Table 28: Sample State Stop-Controlled Intersections – Basic Set of Sign and Marking Improvements
Countermeasure Threshold Crash Level (6 Years) Number of Statewide Crash Intersections Number of Targeted 6 Year Crashes in the Intersections Estimated Number of Improvements1 Construction Costs
($ Million)2		 Fatalities per 100 Crashes Annual Targeted Crash Reduction3 Annual Estimated Fatality Reduction4
Basic Set of Sign and Marking Improvements – State Rural Stop-Controlled Intersections 6 1,221 13,722 977 7.82 1.6 732 11.71
Basic Set of Sign and Marking Improvements – State Urban Stop-Controlled Intersections 30 474 23,795 379 3.03 0.21 1,269 2.67
Total empty cell empty cell empty cell 1,356 10.85 empty cell empty cell 14.38
1 Estimated number of improvements assumes that 80 percent of locations can be improved. Estimated number of improvements is calculated by multiplying the number of statewide crash intersections by the percent of locations that can be improved. For the first row of the table, this calculation is 1,221x0.80 = 977.
2 Construction costs assume an average cost of $8,000 per intersection. Construction costs are calculated by multiplying the estimated number of improvements by the average cost per intersection. For the first row of the table, this calculation is 977x8,000 = $7.82 million.
3 Annual targeted crash reduction uses a CRF of 0.40. Annual targeted crash reduction is calculated by multiplying the average number of targeted crashes per year by the percent of locations that can be improved multiplied by the CRF. For the first row of the table, this calculation is (13,722/6)x0.80x0.40 = 732.
4 Annual estimated fatality reduction is calculated by multiplying the annual targeted crash reduction by the fatalities per 100 crashes and dividing by 100. For the first row of the table, this calculation is (732x1.6)/100 = 11.71.


A trial and error method can be used to develop the straw man outline with an objective to achieve the intersection crash reduction goal with the least costs. Each of the accepted countermeasures can be deployed at levels dependent on the distribution of crashes, severity of crashes (fatalities per 100 crashes), CRF, and unit construction costs. The annual lives saved per $1 million expended can be used as a gauge to determine what levels are appropriate for each countermeasure.

Compile a Summary Straw Man Outline

After all of the individual countermeasure detailed straw man tables are created, the State should develop a summary straw man outline. The summary straw man outline should encompass all of the candidate countermeasures, the impact toward achieving the overall statewide intersection goal, and the costs of improvements. Specific elements include the following:

  • Countermeasures – All of the countermeasures selected in Step 3.
  • Approach – Systematic, comprehensive, or traditional.
  • Number of Statewide Crash Intersections to be Improved – Number of intersections with the crash characteristics that can be impacted by the countermeasure. Transferred from the detailed straw man tables.
  • Construction Cost – Cost for construction of infrastructure countermeasures. Transferred from the detailed straw man tables, as applicable.
  • Enforcement and Education Costs – Costs for enforcement and education, countermeasures. Transferred from the detailed straw man tables, as applicable.
  • Estimated Annual Crashes Reduced – Number of crashes reduced annually. Transferred from the detailed straw man tables.
  • Estimated Annual Incapacitating Injuries Reduced – Number of incapacitating injuries reduced annually. Transferred from the detailed straw man tables, as applicable.
  • Estimated Annual Fatalities Reduced – Number of fatalities reduced annually. Transferred from the detailed straw man tables.

Table 29 shows an example of a summary straw man outline. It is based on a State that established a goal to reduce intersection fatalities by 28 per year by 2012. In this example, the State chose to focus on reducing both fatalities and incapacitating injuries. As a result, the estimated annual incapacitating injuries reduced column is included. In addition, only the countermeasures that the State agreed to implement are included in the table (i.e., other countermeasures are not listed because the State decided not to include them in the intersection safety implementation plan).

Table 29: Sample Summary Straw Man
Countermeasure Approach Number of Intersections to be Improved Construction Costs
($ Million)		 Enforcement, Education and EMS Costs (Annual
$ Thousand)		 Estimated Annual Crashes Reduced Estimated Annual Incapacitating Injuries Reduced Estimated Annual Fatalities Reduced
Basic Set of Sign and Marking Improvements – State Stop-Controlled Intersections (Rural and Urban) Systematic 1,108 8.87 empty cell 1,382 117.7 13.07
Flashing Overhead Intersection Beacons – State Stop-Controlled Intersections (Rural and Urban) Systematic 69 0.69 empty cell 54 4 0.44
J-Turns Modifications on High-Speed Divided Arterials – State Rural Stop-Controlled Intersections Systematic 56 16.8 empty cell 77 17.5 2.87
Basic Set of Sign and Marking Improvements – Local Stop-Controlled Intersections (Rural and Urban) Systematic 236 1.89 empty cell 555 15.1 0.71
Basic Set of Signal and Sign Improvements – State Signalized Intersections (Rural and Urban) Systematic 395 1.92 empty cell 789 28.1 1.52
Basic Set of Signal and Sign Improvements – Local Signalized Intersections (Rural and Urban) Systematic 263 2.63 empty cell 670 19.5 1.51
Change of Permitted and Protected Left-Turn Phase to Protected Only – State Signalized Intersections (Rural and Urban) Systematic 536 2.67 empty cell 819 44 1.49
Change of Permitted and Protected Left-Turn Phase to Protected Only – Local Signalized Intersections (Rural and Urban) Systematic 387 1.94 empty cell 623 23.7 1.27
Advanced Detection Control Systems – State Signalized Intersections (Rural and Urban) Systematic 67 1 empty cell 45 4.2 0.31
New or Upgraded Lighting – State Rural Intersections (Stop-Controlled and Signalized) Systematic 64 3.84 empty cell 49 8.4 1.08
High-Friction Surface – State Intersections (Stop-Controlled and Signalized, Rural and Urban) Systematic 53 2.65 empty cell 86 11.3 1.27
Enforcement-Assisted Lights Systematic 1 City 0.09 0.05 45 2.3 0.11
Corridor 3E improvements on high-speed arterials with very high frequencies of severe intersection crashes Comprehensive 3 Corridors 6 0.3 83 7.5 1.25
Municipal-wide 3E improvements in municipalities with high frequencies of severe intersection crashes Comprehensive 1 City 1 0.1 383 6.6 0.57
Roundabouts empty cell 3 2.4 empty cell 32 3 0.36
Total empty cell empty cell 54.39 0.45 5,692 312.9 27.83


Step 5 Action. Using the intersection crash data analysis package from Step 4, establish threshold crash levels, develop detailed straw man tables for each countermeasure, and compile a summary straw man outline. Use the example data analysis package and straw man outline (found on the FHWA Intersection Safety web page, http://safety.fhwa.dot.gov/intersection/) as a guide.


1 The full example data analysis package and straw man outline can be found on the FHWA Intersection Safety web page (http://safety.fhwa.dot.gov/intersection/).

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