1 / 25

Chapter 11. Highway Traffic Safety: Studies, Statistics, and Programs

Chapter 11. Highway Traffic Safety: Studies, Statistics, and Programs. Chapter objectives: By the end of this chapter students will be able to (we spend one lecture on this chapter):. Describe the trend in accident occurrences Explain approaches to highway safety

laird
Download Presentation

Chapter 11. Highway Traffic Safety: Studies, Statistics, and Programs

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter 11. Highway Traffic Safety: Studies, Statistics, and Programs Chapter objectives: By the end of this chapter students will be able to (we spend one lecture on this chapter): • Describe the trend in accident occurrences • Explain approaches to highway safety • Explain typical data items collected and stored for accidents (through reading) • Determine accident rates given necessary data • Use methods to identify high-accident locations • Conduct properly before-and-after accident analyses • Conduct a site analysis • Describe different safety countermeasures and their cost effectiveness (through reading) Chapter 11

  2. 11.1 Introduction Fatality rates are decreasing but the number of fatalities has plateaued. Year 2008 saw a large decrease. Check: http://www.fhwa.dot.gov/policyinformation/statistics/2008/ and www-fars.nhtsa.dot.gov/ for latest statistics on fatalities. Chapter 11

  3. http://en.wikipedia.org/wiki/List_of_countries_by_traffic-related_death_ratehttp://en.wikipedia.org/wiki/List_of_countries_by_traffic-related_death_rate http://en.wikipedia.org/wiki/List_of_countries_by_population

  4. Fatality and Fatality Rate Trends Fatality rate has decreased because VMT increased. Actual number of death stayed similar for many years. Compare 1995 though 2007 Chapter 11

  5. 11.2 Approaches to highway safety (Page 227-230) Chapter 11

  6. 11.2.6 Planning Actions to Implement Policy Strategies Chapter 11

  7. Traffic planning and operation measures related to highway safety strategies (cont) Chapter 11

  8. Type of Safety Belt Use Laws, by State: As of 2000 Latest Info about Seat Belt Law Chapter 11

  9. 11.3 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 Chapter 11 Example: AIMS (Accident Info Mgmt System) by JMW Engineering

  10. 11.4 Accident statistics • Types of accidents • Numbers of accidents Occurrence • Categories of vehicles • Categories of drivers • No. of deaths • No. of injuries Types of statistics Involvement Severity Chapter 11

  11. Typical accident rates used (p.237) “Bases” are needed to compare the occurrence of accidents at different sites. • Population based: • Area population (25 deaths per 100,000 pop) • No. of registered vehicles (7.5 deaths per 10,000 registered vehicles) • No. of licensed drivers (5.0 deaths per 10,000 licensed drivers) • Highway mileage (5.0 deaths per 1,000 miles) • Exposure based: • VMT (5.0 deaths per 100 million VMT) • VHT (5.0 deaths per 100 million VHT) Severity index: No. of deaths/accident (0.0285 death per accident) No. of injuries/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 Chapter 11

  12. Types of statistical displays The purpose of the display dictates the type of display – temporal, spatial, accident type, etc. Chapter 11

  13. 5% z = 1.645 11.4.4 Identifying high-accident locations (p.237) 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 higher than the average rate of the group. This is a one-tailed test. Why? Example: An intersection has 15.8/1.0 MEV. The mean crash rate for the similar classification group = 12.1 accidents/1.0MVE, SD = 2.5 accidents/1.0MVE. Should an analyst flag this intersection as hazardous at the 95% confidence level? Locations with a higher accident rate than this value would normally be selected for specific study. MEV = Million Entering Vehicles Chapter 11

  14. Determining high-accident locations: Expected value analysis (from Garber & Hoel) • 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” Chapter 11

  15. Example: An intersection with 14 rear-end, 10 LT, and 2 right-angle collisions for 3 consecutive years • 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. Chapter 11

  16. 11.4.5 Statistical analysis of before-after accident data Method 1: Use the Normal Approximation method: z1 = test statistic, 1.96 at the 95% confidence level for a “change”, 1.645 for a “reduction.” fA = No. of accidents in the “after” study fB = No. of accidents in the “before” study Assumption: Accident occurrence is random (Poisson distribution) Mean and variancehave the same value if the sample follows the Poisson distribution (eq 7-15, p.133). When two samples are combined the variances are added. It is assumed the difference in the before and after occurrence is normally distributed. (Accident occurrence itself is Poisson distributed.) This method is however not listed in the current Manual of Transportation Engineering Studies. Chapter 11

  17. 11.4.5 Statistical analysis of before-after accident data (cont) Method 2: The Modified Binomial Test (for a small sample size) – see Fig 7.3 for its typical distribution. Example: Before 14 conflicts were observed at a stop-sign controlled intersection. After the installation of a signal, they observed 7 conflicts. Were the signal effective? Solution: Figure on the right shows that for 14 before conflicts you need a 60% reduction to be significant at the 95% CL. 7/14=50% reduction. So, you cannot reject the null hypothesis (i.e., before = after). Statistically no effect by the signal. Chapter 11

  18. If you can have a control site… Chapter 11

  19. 11.5 Site analysis • Purposes: • Identify contributing causes • Develop site specific improvements • Two types of info: • Accident data • Environment & physical condition data Freedom Blvd. & Bulldog Ave. The first thing you do is visit the site and prepare a condition diagram of the site. Note any potential crash contributing physical conditions. Chapter 11

  20. Site analysis (cont) Then we prepare a collision diagram. Chapter 11

  21. Bike accidents Source:Report for the Provo Bicycle Master Plan Chapter 11

  22. Site analysis (cont) Group accidents by type and answer the following 3 questions, which will lead you to possible countermeasures. • What drivers actions lead to the occurrence of such an accident? • What conditions existing at the location could contribute toward drivers taking such actions • What changes can be made to reduce the chance of such actions occurring in the future? Rear-end collisions: Driver: Sudden stop & Tailgating Environment: Too many accesses and interactions with vehicles in/out of the accesses, bad sight distance, short/long yellow interval, inappropriate location of stop lines, etc. (Table 11-4 is useful for this task) Chapter 11

  23. Chapter 11

  24. 11.6 Development of countermeasures • See Table 11.3 Illustrative programmatic safety approaches. • Table 11.4 Illustrative site-specific accident countermeasures. Chapter 11

  25. Problem 11-3 Chapter 11

More Related