Estimating the Safety Effects of Work Zone Characteristics and Countermeasures: A Guidebook (2018) / Chapter Skim
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From page 5... ...
5 Rationale Sometimes, an agency or contractor will have the need to estimate how many crashes are expected to occur in a work zone, or how many additional crashes are expected above and beyond what would normally occur on that roadway segment over the duration of the work zone. Consider the following examples: • An agency might want to compare the cumulative number of crashes actually occurring at a work zone over time with a predicted rate of crashes expected.
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From page 6... ...
6 Estimating the Safety Effects of Work Zone Characteristics and Countermeasures: A Guidebook • No long-term lane closures present • Median width of 60 ft, which includes the inside shoulder width of 6 ft in both directions • No longitudinal barriers present The more that the pre-work-zone baseline conditions and work zone conditions differ from these ideal attributes, the more that the results of these planning-level analysis techniques will likely deviate from what actually occurs during the work zone. The practitioner must recognize the level of uncertainty inherent in the results obtained from these analyses.
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From page 7... ...
Planning-Level Work Zone Crash Estimation Procedures 7 Six-Lane Facilities = ( )
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From page 8... ...
8 Estimating the Safety Effects of Work Zone Characteristics and Countermeasures: A Guidebook Six-Lane Freeway and Interstate Work Zones Number of work zone crashes expected L n e AADT9.987 1.164ln= × × ( ) − + where, L = length of work zone, miles n = number of years the work zone will require (or number of months/12)
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From page 9... ...
Planning-Level Work Zone Crash Estimation Procedures 9 Computing an Expected Crash Rate per Month During Construction A project engineer plans to monitor crashes occurring during a 2-year, 3-mile Interstate widening construction project. The engineer will extract crashes from the statewide crash database for the roadway segment each month during the project, and wants to be able to detect if the number of crashes that occur becomes unusually high.
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10 Estimating the Safety Effects of Work Zone Characteristics and Countermeasures: A Guidebook 0 5 10 15 20 25 30 35 Cu m ul ati ve N um be r of C ra sh es t o D at e Month of Project Expected Actual 0 9 10 11 121 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 N um be r of W or k Zo ne C ra sh es Ea ch M on th Expected Actual Month of Project 9 10 11 121 2 3 4 5 6 7 8 Figure 6. Expected versus actual work zone crashes using the work zone CMF method for this example.
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From page 11... ...
Planning-Level Work Zone Crash Estimation Procedures 11 actual crashes through 9 months of the project against this estimate (as shown in Figure 7) may or may not have led the engineer to conclude that crashes were becoming excessive relative to what was expected.
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From page 12... ...
12 Estimating the Safety Effects of Work Zone Characteristics and Countermeasures: A Guidebook project based on the data available. Since the only data available for use is the historical crash rate for the roadway segment associated with a lower AADT than that anticipated during the project, the agency would first factor the crash rate for the 2 years of the project using the following ratio of AADT numbers: Year non work zone crash rate crashes mile year crashes mile year 1 - 32.6 120,000 110,000 35.6 = = Year non work zone crash rate crashes mile year crashes mile year 2 - 32.6 130,000 110,000 38.5 = = This factoring process assumed a linear relationship between crashes and AADT, which is typically not true.
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From page 13... ...
Planning-Level Work Zone Crash Estimation Procedures 13 comprehensive crash cost numbers to this value to estimate the road user safety cost savings that could be attributed to this reduction. For example, if the agency typically experiences a crash severity distribution on the facility similar to that shown in Table 1 and used typical crash cost values recommended in the HSM, reducing the project duration would be estimated to yield nearly $900,000 in crash cost savings.
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