Lecture 17: Measurement of Road Safety

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Highway Traffic and Safety Analyses
Lecture 17 Measurement of Road Safety
Purdue University School of Civil
Engineering West Lafayette
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Measurement of Road Safety

• Traffic records system
• Crashes underreporting
• Crash counts vs. crash rates
• Statistical properties of crash counts
• Confidence interval of crash frequency estimate
• Pyramid of traffic events

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Traffic Records Systemhttp//www.dottrcc.gov/

• Crash File with data on the time, environment,
  and circumstances of a crash identification of
  the vehicles, drivers, cyclists, occupants, and
  pedestrians involved and documentation of crash
  consequences (fatalities, injuries, property
  damage and violations charged) with the data tied
  to a location reference system. Report form
  (show).
• Commercial Motor Vehicle Crash File which uses
  uniform data definitions and collects information
  on the vehicle configuration, cargo body type,
  hazardous materials, information to identify the
  motor carrier, as well as information on the
  crash (States are encouraged to use available
  information systems to cross-reference commercial
  vehicle citations for violations of Federal and
  State commercial vehicle safety regulations).
• Roadway File with information about roadway
  location, identification, and classification as
  well as a description of a road's total physical
  characteristics, which are tied to a location
  reference system. This file should also contain
  data for normalizing purposes, such as miles of
  roadway and annual average daily traffic (AADT).
  

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Traffic Records System

• Emergency Medical Services (EMS) file with
  emergency care and victim outcome information
  about ambulance responses to crashes, e.g.,
  emergency care unit, care given, injury data, and
  times of EMS notification and arrival
  information on emergency facility and hospital
  care, including Trauma Registry data and medical
  outcome data relative to crash victims receiving
  rehabilitation and for those who died as the
  result of the crash.
• Citation/Conviction File which identifies the
  type of citation and the time, date, and location
  of the violation the violator, vehicle and the
  enforcement agency and adjudication action and
  results, including court of jurisdiction (an
  Enforcement/Citation File could be maintained
  separate from a Judicial/Conviction File) and
  fines assessed and collected.

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Traffic Records System

• Driver File or driver history record of licensed
  drivers in the State, with data on personal
  identification and driver license number, type of
  license, license status (suspended or revoked),
  driver restrictions, driver convictions for
  traffic violations, crash history, driver control
  or improvement actions, and safety education
  data.
• Vehicle File with information on identification,
  ownership and taxation, and vehicle inspection
  (where applicable).

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Traffic Records System

• ! Provisions for file linkage through common data
  elements between the files or through other
  consistent means performance level data as part
  of the traffic records system demographic data
  to normalize or adjust for exposure when
  analyzing the various data in the files and
  provisions for the use of cost data relative to
  amounts spent on countermeasure
  programs and the costs of fatalities, injuries
  and property damage.

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Underreporting of CrashesHauer and Hakkert,
Extent and some Implications of Incomplete
Accidents Reporting, Transportation Research
Record 1185
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Crash-based Measures of Safety

• Annual crash counts c1, c2cn
• Estimate of crash frequency (expected annual
  count)

• Expected crash rate a / Exposure
• Exposure annual volume, annual VMT, etc.

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Crash Counts vs. Crash Rates
Crash Rate BEFORE
Crash Rate AFTER
Crashes
VMT
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Statistical Properties of Crash Counts
Poisson variability of annual counts
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Gamma distribution
0.05
0.05
au
al
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Confidence Interval of Frequency
EstimateGamma-based Estimation
90 confidence interval
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Confidence Interval of Frequency
EstimateGamma-based Estimation

• c 10 crashes, n 3 years,
• 90 confidence level
• Expected crash frequency
• a c/n10/3 3.33 crashes/year
• Lower limit of the expected crash frequency
• al GAMMAINV(0.05, 10, 1/3)
• al GAMMAINV (0.05, 10, 0.33) 1.81
  crashes/year
• Upper limit of the expected crash frequency
• au GAMMAINV (0.95, 10, 1/3)
• au GAMMAINV (0.95, 10, 0.33) 5.24
  crashes/year

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Confidence Interval of Frequency Estimate
Nicholson, The Estimation of Accident Rates and
Countermeasure Effectiveness, Traffic Engineering
and Control, October 1987, pp. 518-523
a3.3n 1
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Confidence Interval of Frequency Estimate
Nicholson, The Estimation of Accident Rates and
Countermeasure Effectiveness, Traffic Engineering
and Control, October 1987, pp. 518-523
a3.3n 3
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Confidence Interval of Frequency Estimate
Nicholson, The Estimation of Accident Rates and
Countermeasure Effectiveness, Traffic Engineering
and Control, October 1987, pp. 518-523
a3.3n 5
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Confidence Interval of Frequency Estimate
Nicholson, The Estimation of Accident Rates and
Countermeasure Effectiveness, Traffic Engineering
and Control, October 1987, pp. 518-523
a3.3n 10
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Normal
Poisson
Mean 1.8
Mean 27.0
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Confidence Interval of Frequency EstimateNormal
Approximation
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Confidence Interval of Frequency EstimateNormal
Approximation

• Normal distribution
• c total crash count during n years
• al lower bound of the confidence interval for
  the mean
• au upper bound of the confidence interval for
  the mean
• 100k confidence level

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Confidence Interval of Frequency EstimateNormal
Approximation Example

• c 10 crashes, n 3 years,
• 90 confidence level, k 0.90
• Expected crash frequency
• a c/n10/3 3.33 crashes/year
• Lower limit of the expected crash frequency
• al NORMINV(0.05, 10/3, 101/2/3)
• al MORMINV(0.05, 3.33, 1.05) 1.60
  crashes/year
• Upper limit of the expected crash frequency
• au NORMINV(0.95, 10/3, 101/2/3)
• au MORMINV(0.95, 3.33, 1.05) 5.07
  crashes/year

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Confidence Interval of Frequency
EstimateComparison

• c 10 crashes, n 3 years,
• 90 confidence level

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Confidence Interval of Crash Rate

• Estimate the confidence interval of crash
  frequency
• Convert the interval limits to crash rates
• Assumption negligible error of exposure estimates

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Pyramid of Traffic Events
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Indiana Police Crash Report Form Full form Back