Lec 32, Ch5, pp.131-153: Highway Safety Improvement Program (objectives)

1. 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) • Learn typical accident data analysis methods and a method to identify locations with accident rates higher than average locations • Learn a method to identify hazardous locations and elements “Evaluate crash data, redesign and reconstruct the highway system where the potential for high crash rates exists”

2. What we discuss in class today… (emphasis on yellow-color topics) • Current accident statistics • Components of HSIP by FHWA • Accident rates typically used • Methods of summarizing accident data • A method to identify high-accident-rate locations: expected value analysis • A method to identify hazardous locations and elements

3. POW! Wrong jump!

9. Almost 29% of the fatal accidents involved speeding in 2000 (about 12,350 lost their lives).

10. Component of the Highway Safety Improvement Program (HSIP) by FHWA Need estimates of the effectiveness of safety design features

11. Accident data collection and record systems • One of the most basic functions of traffic engineering is keeping track of the physical inventory. Collision diagram Accident spot map Example: AIMS (Accident Info Mgmt System) by JMW Engineering

12. Types of statistical displays The purpose of the display dictates the type of display – temporal, spatial, accident type, etc.

13. Methods of summarizing accident data • Types of accidents • Numbers of accidents • Time period Occurrence • Categories of vehicles • Categories of drivers • Environmental conditions • No. of deaths • No. of injuries • No. of POD Types of statistics Severity Involvement

14. Typical accident rates used “Bases” are needed to compare the occurrence of accidents at different sites. • Population based: • Area population • No. of registered vehicles • No. of licensed drivers • Highway mileage • Exposure based: • VMT • VHT Severity index: No. of deaths/accident • Typical basic accident rates: • general accident rates describing total accident occurrence • fatality rates describing accident severity • involvement rates describing the types of vehicles and drivers involved in accidents

15. Sample accident rates in pages 139-140 (rates are usually annual values) Rate per million of entering vehicles (for intersections): Rate per 100 million vehicle miles (for highway segments): (Thousands) (See Example 5-1 and 5-2)

16. Determining high/low-accident locations: Expected value analysis (p.140) • Note this method is used only to compare sites with similar characteristics. 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 “Under-represented” “Over-represented”

17. Example 5-3 (modified): An intersection with 14 rear-end, 10 LT, and 2 right-angle collisions for 3 consecutive years (p.141) • Check about rear-end collisions Rear-end collisions are over-represented at the study site at 95% confidence level, since 14 > 10.34. • Check about LT collisions LT collisions are not over-represented or under-represented at the study site at 95% confidence level, since 0.88<10 < 12.92. • Check about right-angle collisions Right-angle collisions are under-represented at the study site at 95% confidence level, since 2 < 2.4.

18. Identifying hazardous locations and elements • Crashes happen randomly and are “rare events.” You cannot identify hazardous locations simply on the basis of the number of crashes. • A technique known as the critical crash rate factor method is one method used to identify hazardous locations. This method incorporates the traffic volume to determine if the crash rate at a particular location is significantly higher than the average for the type of facility. • We compute first the statewide CR value for similar types of roadway, which works as a “threshold.” 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

19. Identifying hazardous locations and elements (cont) • Once the critical crash rate is computed, compute segment crash history in terms of PDO equivalents. • Then compute Crash Ratio as defined below: Segment crash history Crash Ratio = Statewide crash history • If the resulting accident ratio is greater than 1.0, then a safety problem is likely to exist. (Review Example 5-4)