Estimating the Safety Effects of Work Zone Characteristics and Countermeasures: A Guidebook (2018) / Chapter Skim
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Pages 14-20
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From page 14... ...
14 Rationale Oftentimes, practitioners developing construction plans have options for accommodating traffic through the various phases or stages of the project. These alternatives can include factors such as whether or not to (a)
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Using CMFs to Evaluate Alternative Work Zone Design Features, Operating Strategies, and Safety Countermeasures 15 safety effects are to be compared. Each feature, strategy, or countermeasure that differentiates between each alternative is defined.
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16 Estimating the Safety Effects of Work Zone Characteristics and Countermeasures: A Guidebook of available data and methods for estimating baseline work zone crashes on most facilities. Consequently, the accuracy of the results obtained using this methodology will only be as accurate as the baseline crash estimates used.
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Using CMFs to Evaluate Alternative Work Zone Design Features, Operating Strategies, and Safety Countermeasures 17 For comparison purposes, the normal number of crashes expected to occur on this section of roadway on these nights if no work zone is present would be 21.28 crashes divided by the work zone CMF of 1.61, or 13.22 crashes. Thus, the nighttime lane closures are predicted to result in 8.06 additional crashes (i.e., 21.28 – 13.22 = 8.06)
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18 Estimating the Safety Effects of Work Zone Characteristics and Countermeasures: A Guidebook that occur when a queue warning system is implemented is also reduced relative to a no-system condition, which would further increase the crash cost savings that might be achieved (6)
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Using CMFs to Evaluate Alternative Work Zone Design Features, Operating Strategies, and Safety Countermeasures 19 It is also assumed that work will occur Monday through Friday but not on weekends. Therefore, the lane closure CMF for Part 1 is computed as follows: WZCMF days days wk days days wk Lane closures Part 5 7 1.66 0.30 1.0 0.7 2 7 1.0 1.14, 1 ( )
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20 Estimating the Safety Effects of Work Zone Characteristics and Countermeasures: A Guidebook Therefore, the total estimate of crashes for Alternative 2 would be as follows: 2.00 0.12 2.122Crashes crashesAlt = + = Step 5. Compute differences in crash estimates between alternatives The final step in the process is to subtract the crash estimates from each alternative to determine the expected differences in crashes: 2.28 2.12 0.161 2Expected Crash Difference crashesAlt Alt = − =− Thus, Alternative 1 is expected to result in an additional 0.16 crashes during the project, despite being of slightly shorter duration.
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