Lec 32, Ch5, pp.131-153: Highway Safety Improvement Program (objectives). Learn the components of FHWA’s Highway Safety Improvement Program Know typical data collection and data maintenance methods (through reading)
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“Evaluate crash data, redesign and reconstruct the highway system where the potential for high crash rates exists”
Almost 29% of the fatal accidents involved speeding in 2000 (about 12,350 lost their lives).
Need estimates of the effectiveness of safety design features
Accident spot map
Example: AIMS (Accident Info Mgmt System) by JMW Engineering
The purpose of the display dictates the type of display – temporal, spatial, accident type, etc.
Types of statistics
“Bases” are needed to compare the occurrence of accidents at different sites.
No. of deaths/accident
Rate per million of entering vehicles (for intersections):
Rate per 100 million vehicle miles (for highway segments):
(See Example 5-1 and 5-2)
H0: Accident rate at the location under consideration in the group is equal to the average rate of the group.
H1: Accident rate at the location under consideration in the group is not equal to the average rate of the group (In another words, we are trying to find whether the site under study is “unusual” or not. We are not specifically proving it is “over-represented” or not.)
z = 1.96 for the 95% confidence level
Locations with a higher accident rate than this value would normally be selected for specific study.
Not over-represented or under-represented
Rear-end collisions are over-represented at the study site at 95% confidence level, since 14 > 10.34.
LT collisions are not over-represented or under-represented at the study site at 95% confidence level, since 0.88<10 < 12.92.
Right-angle collisions are under-represented at the study site at 95% confidence level, since 2 < 2.4.
CR = critical crash rate, per 100 million VMT or per million entering vehicles
AVR = average crash rate for the facility type (in terms of PDO crashes; a multiplicative factor is used to convert the impact of death/injury accidents to PDO accidents)
TF = test factor, Z score (remember this needs to be a one-way analysis because we try to find a “significantly higher” or “hazardous” location) for a given confidence level,
TB = traffic base, 100 million VMT or million entering vehicles
Segment crash history
Crash Ratio =
Statewide crash history
(Review Example 5-4)