Chapter 111

Chapter 11. Highway Traffic Safety: 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 one lecture on this chapter):

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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.

http://en.wikipedia.org/wiki/List_of_countries_by_traffic-related_death_rate

http://en.wikipedia.org/wiki/List_of_countries_by_population 3

Fatality and Fatality Rate Trends

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Fatality rate has decreased because VMT increased.Actual number of death stayed similar for many years. Compare 1995 though 2007

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11.2 Approaches to highway safety11.2.1 Exposure control

Strategies that reduce the number of VMT of travel by motorists. (TDM measures. Park & ride, car-pool, public transit, etc.)

11.2.2 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.

11.2.3 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.

11.2.4 Injury control Focuses on crash survivability of occupants. Better vehicle design. Seat belts use, child seats, padded instrument panels, air bags, shatter proof glass, etc.

11.2.5 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

11.2.6 Planning actions to implement policy strategies

Consider reduction of exposure, accident risk, and injury control in planning and design stages (See Table 11.2 for examples).

11.2.7 National policies

Imposition of broad policy initiatives and programs. Inspection programs, 21-years old drinking age, DWI 0.10.08, etc.

(Page 227-230)

11.2.6 Planning Actions to Implement Policy Strategies

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Traffic planning and operation measures related to highway safety strategies (cont)

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Chapter 118

Type of Safety Belt Use Laws, by State: As of 2000

Latest Info about Seat Belt Law

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11.3 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

Collision diagram

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11.4 Accident statistics

Occurrence

Involvement

Severity

Types of accidents

Numbers of accidents

Categories of vehicles

Categories of drivers No. of deaths

No. of injuriesTypes of statistics

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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)

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

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Types of statistical displaysThe purpose of the display dictates the type of display – temporal, spatial, accident type, etc.

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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?

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:

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?

MEV = Million Entering Vehicles

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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

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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.

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11.4.5 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.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.)

BA

AB

BA

AB

ff

ff

ff

ff

s

xxz

0)(1

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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.

If you can have a control site…

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Chapter 1119

11.5 Site analysis

The first thing you do is visit the site and prepare a condition diagram of the site. Note any potential crash contributing physical conditions.

Purposes:

Identify contributing causes

Develop site specific improvements

Two types of info:

Accident data

Environment & physical condition data

Freedom Blvd. & Bulldog Ave.

Chapter 1120

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

Bike accidents

Chapter 1121

Source:Report for the Provo Bicycle Master Plan

Chapter 1122

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)

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11.6 Development of countermeasures

See Table 11.3 Illustrative programmatic safety approaches.

Table 11.4 Illustrative site-specific accident countermeasures.

Problem 11-3

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