Chapter 10' Accidents: Studies, Statistics, and Programs

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Chapter 10. Accidents Studies, Statistics, and
Programs
Chapter objectives By the end of this chapter
students will be able to (we spend 1 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)

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10.1 Introduction

• Fatality rates are decreasing but the number of
  fatalities has plateaued.

Check http//www.fhwa.dot.gov/policyinformation/s
tatistics/2007/ and www-fars.nhtsa.dot.gov/
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10.2 Approaches to highway safety
(Page 239-242)
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Type of Safety Belt Use Laws, by State As of
2000
Latest Info about Seat Belt Law
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10.3 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
Example AIMS (Accident Info Mgmt System) by JMW
Engineering
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10.4 Accident statistics

• Types of accidents
• Numbers of accidents

Occurrence

• No. of deaths
• No. of injuries

• Categories of vehicles
• Categories of drivers

Types of statistics
Involvement
Severity
http//www-nrd.nhtsa.dot.gov/departments/nrd-30/nc
sa/STSI/49_UT/2008/49_UT_2008.htm/
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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)

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

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Types of statistical displays
The purpose of the display dictates the type of
display temporal, spatial, accident type, etc.
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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?
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?
Locations with a higher accident rate than this
value would normally be selected for specific
study.
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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
Over-represented
Under-represented
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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

Rear-end collisions are over-represented at the
study site at 95 confidence level, since 14 gt
10.34.

• Check about LT collisions

LT collisions are not over-represented or
under-represented at the study site at 95
confidence level, since 0.88lt10 lt 12.92.

• Check about right-angle collisions

Right-angle collisions are under-represented at
the study site at 95 confidence level, since 2 lt
2.4.
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10.4.5 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 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.)
This method is however not listed in the current
Manual of Transportation Engineering Studies.
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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. 7/1450 reduction.
So, you cannot reject the null hypothesis (i.e.,
before after). Statistically no effect by the
signal.
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10.5 Site analysis

• Purposes
• Identify contributing causes
• Develop site specific improvements

• Two types of info
• Accident data
• Environment physical condition data

The first thing you do is visit the site and
prepare a condition diagram of the site.
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Site analysis (cont)
Then we prepare a collision diagram.
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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)
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10.6 Development of countermeasures

• See Table 10.3 Illustrative programmatic safety
  approaches.
• Table 10.4 Illustrative site-specific accident
  countermeasures.