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Lec 33, Ch.5, pp.147-164: Accident reduction capabilities and effectiveness of safety design features (Objectives) . Learn what’s involved in safety engineering studies Learn how to compute accident reduction capabilities of countermeasures
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Need estimates of the effectiveness of safety design features
The first thing you do is visit the site and prepare a condition diagram of the site.
Group accidents by type and answer the following 3 questions, which will lead you to possible countermeasures. See Table 5.3.
Driver: Sudden stop & Tailgating
Environment: Too many accesses and interactions with vehicles in/out of the accesses (drive ways), bad sight distance, short/long yellow interval, inappropriate location of stop lines (against driver expectancy), etc.
N = expected number of crashes if countermeasure is not implemented and if the traffic volume remains the same.
CR = 0.3, ADT before = 7850, ADT after = 9000, No. of specific types of crash occurring per year = 12, 14, 13 for the same 3 years where ADT average values were computed.
Avg no. of crashes/year = (12+14+13)/3=13
Crashes prevented = 13 x 0.3 x (9000/7850) = 4.47 say, 4 accidents
CR = overall crash reduction factor for multiple mutually exclusive improvements at a single site
CRi = crash reduction factor for a specific countermeasure i
m = number of countermeasures at the site
CR1 = 0.40, CR2 = 0.28, and CR3 = 0.2. Determine the overall CR factor. Note that countermeasures are ordered in the descending order of their accident reduction factor values.
CR = 0.4 + (1 – 0.4)*0.28 + (1 – 0.4)(1 – 0.28)*0.2
Access Control: Defined as “some combination of at-grade intersections, business and private driveways, and median crossovers”
More access control Less accidents
More access, higher crash rates
(Note that the access control section of the chapter does not give CR values.)
(Note that the alignment section of the chapter does not give CR values.)
Table 5.11 Ratio of Cross Section Related Crashes to Total Crashes on Two-Lane Rural Roads
F = fatal accidents
I = injury accidents
Find the expected number of accidents reduced: