Chapter 10 1 Chapter 10. Accidents: Studies, Statistics, and Programs 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 objectives: By the end of this chapter students will be able to (we spend 1 lecture on this chapter):
Transcript
Chapter 101
Chapter 10. Accidents: Studies, Statistics, and Programs
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 objectives: By the end of this chapter students will be able to (we spend 1 lecture on this chapter):
Chapter 102
10.1 Introduction
Fatality 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 103
10.2 Approaches to highway safety
10.2.1 Exposure control
Strategies 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 prevention
Accident 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 modification
Easy 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 control
Federal 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, etc
10.2.7 National policies
Imposition of broad policy initiatives and programs. Inspection programs, 21-years old drinking age, DWI 0.10.08, etc.
(Page 239-242)
Chapter 104
Type of Safety Belt Use Laws, by State: As of 2000
Latest Info about Seat Belt Law
Chapter 105
10.3 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 map
Typical 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/accident
Chapter 108
Types of statistical displaysThe purpose of the display dictates the type of display – temporal, spatial, accident type, etc.
Chapter 109
Determining 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 = 1.645
5%
645.11
zs
xxz xsx 645.11
Locations 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 1010
Determining 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 %5.2
zsx zsx x
Chapter 1011
Example: 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 over-represented or under-represented at the study site at 95% confidence level, since 0.88<10 < 12.92.
Control site Rear-end
LT collisions
Right-angle
1 8 11 4
2 5 12 5
3 7 4 3
4 8 5 6
5 6 8 7
6 8 3 8
7 9 4 4
8 10 9 5
9 6 7 6
10 7 6 7
Mean 7.40 6.90 5.5
SD 1.5 3.07 1.58
Check about right-angle collisions 65.104.296.158.15.505.0 toEV Right-angle collisions are under-represented at the study site at 95% confidence level, since 2 < 2.4.
Chapter 1012
10.4.5 Statistical analysis of before-after accident data
BA
BA
ff
ffz
1
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
This method is however not listed in the current Manual of Transportation Engineering Studies.
Assumption: Accident occurrence is random (Poisson distribution)
Mean and variance have the same value if the sample follows the Poisson distribution (eq 7-15, p.144). 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.)
BA
AB
BA
AB
ff
ff
ff
ff
s
xxz
0)(1
Chapter 1013
Statistical analysis of before-after accident data (cont)Method 2: The Modified Binomial Test
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 1014
10.5 Site analysis
The first thing you do is visit the site and prepare a condition diagram of the site.
Purposes:
Identify contributing causes
Develop site specific improvements
Two types of info:
Accident data
Environment & physical condition data
Chapter 1015
Site analysis (cont)Then we prepare a collision diagram.
Chapter 1016
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 10-4 is useful for this task)
Chapter 1017
10.6 Development of countermeasures
See Table 10.3 Illustrative programmatic safety approaches.
