Chapter 10. Accidents: Studies, Statistics, and Programs10.1 Introduction10.2 Approaches to highway safetyType of Safety Belt Use Laws, by State: As of 200010.3 Accident data collection and record systems10.4 Accident statisticsTypical accident rates usedTypes of statistical displaysDetermining high-accident locations (p.251)Determining high-accident locations: Expected value analysis (from Garber & Hoel)Example: An intersection with 14 rear-end, 10 LT, and 2 right-angle collisions for 3 consecutive years10.4.5 Statistical analysis of before-after accident dataStatistical analysis of before-after accident data (cont)10.5 Site analysisSite analysis (cont)Slide 1610.6 Development of countermeasuresChapter 101Chapter 10. Accidents: Studies, Statistics, and ProgramsDescribe the trend in accident occurrencesExplain approaches to highway safetyExplain typical data items collected and stored for accidents (through reading)Determine accident rates given necessary data Use methods to identify high-accident locationsConduct properly before-and-after accident analysesConduct a site analysisDescribe different safety countermeasures and their cost effectiveness (through reading)Chapter objectives: By the end of this chapter students will be able to (we spend 1 lecture on this chapter):Chapter 10210.1 IntroductionFatality rates are decreasing but the number of fatalities has plateaued.Check: http://www.fhwa.dot.gov/policyinformation/statistics/2007/ and www-fars.nhtsa.dot.gov/Chapter 10310.2 Approaches to highway safety10.2.1 Exposure controlStrategies that reduce the number of VMT of travel by motorists. (TDM measures. Park & ride, car-pool, public transit, etc.)10.2.2 Accident risk control/accident preventionAccident prevention implies actions that reduce the number of accidents that occur for a given demand level. Accident risk control incorporates this, but also includes measures that reduce the severity of an accident when it occurs. (Roadside design, license suspension, etc) Embed safety in facility design: alignment, roadside, median barriers, gore areas, etc.10.2.3 Behavior modificationEasy to say, hard to do. Affecting mode choice is a major behavior modification action that is hard to successfully achieve. Or making motorists understand following a speed limit is a benefit to them.10.2.4 Injury controlFederal and state programs that address traffic safety on a policy level, like state vehicle inspection programs, speed limits, 21 year-old drinking age, state DWI programs, seat belts, air bags, etc.10.2.5 Post-injury management* Death within minutes, Death within one to two hours, Death within 30 days. Impact on Death within one to two hours by imoroved medical response, trained medical technicians, improved equipment, etc10.2.7 National policiesImposition of broad policy initiatives and programs. Inspection programs, 21-years old drinking age, DWI 0.10.08, etc.(Page 239-242)Chapter 104Type of Safety Belt Use Laws, by State: As of 2000 Latest Info about Seat Belt LawChapter 10510.3 Accident data collection and record systems One of the most basic functions of traffic engineering is keeping track of the physical inventory.Example: AIMS (Accident Info Mgmt System) by JMW EngineeringAccident spot mapCollision diagramChapter 10610.4 Accident statisticsOccurrenceInvolvement Severity Types of accidents Numbers of accidents Categories of vehicles Categories of drivers No. of deaths No. of injuriesTypes of statisticshttp://www-nrd.nhtsa.dot.gov/departments/nrd-30/ncsa/STSI/49_UT/2008/49_UT_2008.htm/Chapter 107Typical accident rates used“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)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 BaseScaleTotalRate Severity index: No. of deaths/accident (0.0285 death per accident)No. of injuries/accidentChapter 108Types of statistical displaysThe purpose of the display dictates the type of display – temporal, spatial, accident type, etc.Chapter 109Determining high-accident locations (p.251)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?z = 1.6455%645.11 zsxxz xsx 645.11Locations with a higher accident rate than this value would normally be selected for specific study.Example: Highway Section 33 has 210 accident/100MVMT. The mean accident rate for the similar classification group = 89 accidents/100MVMT, SD = 64 accidents/100MVMT. Should an analyst flag Section 33 as hazardous? With the 95% confidence level?Chapter 1010Determining high-accident locations: Expected value analysis (from Garber & Hoel)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.)ZSxEV Locations with a higher accident rate than this value would normally be selected for specific study. Note this method is used only to compare sites with similar characteristics.z = 1.96 for the 95% confidence level“Over-represented”“Under-represented”Not over-represented or under-represented%5.2%5.2zsx zsx xChapter 1011Example: An intersection with 14 rear-end, 10 LT, and 2 right-angle collisions for 3 consecutive years Check about rear-end collisions 34.1046.496.15.140.705.0 toEV Rear-end collisions are over-represented at the study site at 95% confidence level, since 14 > 10.34. Check about LT collisions 92.1288.096.107.390.605.0 toEV LT collisions are not
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