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Traffic Safety (1) & (2). Lecture 23 & 24. Definition of Crash. What is an motor vehicle crash?

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Traffic safety 1 2

Traffic Safety (1) & (2)

Lecture 23 & 24

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Definition of Crash

  • What is an motor vehicle crash?

    A motor vehicle accident is “Any incident in which bodily injury or damage to property is sustained as a result of the movement of a motor vehicle, or of its load while the motor vehicle is in motion. ”

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Definition of Safety

The safety of an entity is:

the number of accidents (crashes) or accident consequences, by kind and severity, expected to occur on the entity during a specified period.

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Crash Data

  • Crashes are complex events caused by many different factors. These factors could be divided in three categories:

    a. Drivers, occupant, pedestrian (85%)

    b. Road (10%)

    c. Vehicle (5%)

  • In order to develop appropriate corrective measures, it is important to determine why crashes happen based on details records of crash occurrences.

  • It is necessary to have information for crashes regarding:

    • Location

    • Type

    • Severity

    • Frequency

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Collection of Crash Data

  • Crash data cannot be objectively observed similar to other traffic stream parameters.

  • Crash data is collected from secondary sources: motorist and police crash reports

    a. Individual crash report: required by law for all crashes with a property damage equal or above certain level.

    b. Police report: for more serious crashes investigated by a police officer.

  • Many of these crashes may involve correctable design, control, or operational features that need to be identified by analyzing the crash data.

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Issues in Collecting Crash Data

  • A system of gathering, storing and retrieving crash information in a useful form must be carefully designed and monitored to properly evaluate and correct traffic safety deficiencies.

  • An important consideration in crash reporting is the high under-reporting of crashes, specially all property damage only (PDO) crashes below the legal reporting limit.

  • Traffic engineers lack the information on these unreported crashes.

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Crash Reports (1)

  • The information gathered in crash reports include:

    • Time and location of the crash,

    • Characteristics of the roadway/intersection,

    • Detailed information about vehicles

    • People involved in the crash,

    • Prevailing environmental conditions at the time of the crash,

    • Description of the occurrence of the crash,

    • A sketch of the crash, and

    • Others.

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Crash Reports (2)

  • Crash Severity

    a. Crash severity is determined by using the most severe consequence to a person involved in the crash (driver, passenger or non-motorist).

    b. Injury severity recorded under six categories:

    1 – None

    2 – Possible injury

    3 – Non-incapacitating injury

    4 – Incapacitating injury

    5 – Fatal (within 90 days)

    6 – Non traffic fatality

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Crash Reports (3)

  • Crash Type

    a. Type of crash is determined based on the first harmful event

    b. There are 26 different types of crashes in the crash database maintained by FDOT

    • Rear-End

    • Angle

    • Sideswipe

    • Pedestrian

    • ….

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Crash Reports (4)

  • Police crash report forms are generally sent and stored in three different locations:

    a. A copy goes to state motor vehicle department for entry into the state’s computer crash data system

    b. A copy goes to central filing location for the municipality or district in which the crash occurred,

    c. A copy is retained by the officer as a reference for possible court testimony.

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Computer Based Crash Database

  • All states, many large municipalities and counties maintain computer based crash record systems that supplement manual reporting systems (Crash Analysis Reporting System – FDOT CAR)

  • Advantage – capability to maintain a large number of crash records, that can be access very quickly and related to other traffic data such as ADT at specific locations. Easy to separate subsets of crash data depending on the information needed.

  • Disadvantage – limited to information that can be reduced to alphanumeric codes. The detail of individual accident diagrams and descriptions are lost.

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How to Use Crash Database – Time Frame

  • Two factors of deciding a time frame. Data from outside the selected time period will not be considered.

    (1) Desire for larger sample size

    (2) Desire for time frames within which conditions have not changed too much.

  • Once the study time frame is decided, crash data should be separated out corresponding to that time period.

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How to Use Crash Database– Study Locations

  • Spots and Sections:

    • Spots are short segments of the roadway that help identify the problem point location, such as intersections, curves, and short bridges. Geometric and other features at a spot should be noticeably different from surrounding spots. Recommended spot lengths are from 0.2 to 0.3 miles.

    • Sections are longer, rather homogeneous sections of the roadway. Recommended section lengths are from 1 to 2 miles.

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How to Use Crash Database– Others

  • Precise definitions for the limits are required for analyzing intersection crashes. A fixed distance of 100-200 feet along each of the approach leading to the intersection is used in most of the cases.

  • Reducing crash data based on other variables may be required based on the purpose of the study. Interest may be on the following:

    • The crash type

    • Time of the day

    • Months of the year

    • Weather

    • Types of vehicles

    • Driver conditions

    • Age of the drivers, etc.

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Crash Frequency & Crash Rate (1)

1. Crash Frequency is the number of crashes occurred in a given period (one year).

2. Crash Rate:

Crash Rate =

Why not use absolute value?

People are exposed to transportation risks when they travel. Assuming that everything else remains the same, more travel there is, the more risk people will incur. In this sense, it will not be proper to use absolute number to compare different situations.

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Crash Frequency & Crash Rate (2)

  • One assumption behind using crash rate is that the crash frequency is proportional to exposure. This assumption may not always hold true.

  • Empirical evidence shows that the relationship between expected crash frequency and flow rate is usually not a linear one.

  • Drivers behave differently in sparse and heavy traffic and the frequency of single and multi-vehicle crashes depends on traffic flow, speed and density.

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Population and Exposure Based Crash Rate

Population based rates:

- Area population

- Number of driver licensed population

- Number of motor vehicle registered

- Highway (roadway) mileage

Exposure based rates:

- Vehicles miles of travel (VMT)

- Vehicles hours of travel (VHT)

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Basic Crash Rate (1)

  • 1. Crash involvement rate:

  • R = crash rate per 100 million vehicle miles traveled

  • N = total number of drivers of vehicles involved in crashes (usually in 1 year)

  • V = total vehicle miles traveled

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Basic Crash Rate (2)

2. Death/Fatality rate based on population

R = death/fatality Rate per 100,000 people

B = total number of traffic fatalities

P = population

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Basic Crash Rate (3)

3. Death/Fatality Rate Based on Driver-Licensed Population

R = death/fatality rate per 10,000 licensed drivers

B = total number of fatalities

D = driver licensed population

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Basic Crash Rate (4)

4. Death/fatality rate based on registered-vehicle

R = death/fatality rate per 10,000 registered-vehicles

B = total number of fatalities

M = total number of registered motor vehicles

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Basic Crash Rate (5)

5. Death/fatality rate based on VMT

R = death/fatality rate per 100 million VMT

B = total number of fatalities

V = total number of vehicles miles traveled

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Basic Crash Rate (6)

  • 6. Crash rate per mile

    • R = crash rate per mile

    • A = total number of crashes on the considered section (usually for 1 year)

    • L = length of the section

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Basic Crash Rate (7)

  • 7. Crash Rate Per Million Involved Vehicles Per Mile

  • R = crash rate per million involved vehicles per mile

    • A = total number of crashes on the roadway section

    • T= the time frame of the analysis (years);

    • V= the average ADT volume of the segment at three years time period; and

    • L= the length of the selected roadway segment (miles).

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City X

Fatalities 75

Fatal Crashes 60

Injury Crashes 300

Property Damage Only (PDO) crashes 2000

Total involvements 4100

Vehicles-miles 1,500,000,000

Registered Vehicles 100,000

Licensed Drivers 150,000

Area population 300,000

R= 25 deaths/fatalities per 100,000 people

R= 5 deaths/fatalities per 10,000 licensed drivers

R= 7.5 deaths/fatalities per 10,000 registered-vehicles

R= 5 deaths/fatalities per 100 million VMT

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Safety Concerns

  • Roadways

    1. Intersection Safety

    2. Highway and Street Safety

    3. Merging Area Safety

    4. Special Safety Concerns

    a. School bus safety

    b. Pedestrian and bicycles safety

    c. Safety of special population groups

    d. Terrorism and transportation security

  • Air Transportation

    1. Runway Incursion and Collision

    2. Mid-air Collision

    3. Air Traffic Controller Operational Error

    4. Special Safety Concerns (Terrorism,…)

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Identifying and Eliminating Hazards

  • The standard procedure consists of the following steps:

    1. Identify hazardous locations based on reported crash data

    2. Obtaining detailed design problems in the hazardous locations by conducting engineering studies

    3. Identify possible countermeasures for hazardous locations

    4. Predicting the effect of potential countermeasures in terms of reduced number of crashes or severity of crashes

    5. Implementing countermeasures with the highest net benefits on investment,

    6. Evaluating the effectiveness of the countermeasures after implementation.

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Identifying Crash Prone Locations

  • Spots maps: created by marking the location of each relevant crash on a map. Example: map hung on a wall, computer graphics (GIS technique), etc. could be color-coded. Very useful for specialized situations such as pedestrian crashes.

  • Obtain the Crash frequency or crash rate of each location

  • Identify those with crash frequencies or crash rates that are significantly higher than the average for the jurisdiction under study.

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Example: The locations with crash rates in the

highest 5% of the normal distribution

In a one-tailed test, the value of z (on the standard normal distribution) for Probability (z) < 0.95 is 1.645.

xi= crash rate at the location under consideration

= average crash rate for locations within the jurisdiction under study

s = standard deviation of crash rates for locations within the jurisdiction under study


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Identifying Prominent Cluster of Crashes

  • Main tool used by many agencies to identify the clusters: Collision diagram – a schematic, not to scale graphical representation of the crash pattern at a particular location.

  • Each accident is plotted separately on the approach and near the place where the first harmful event is said to have occurred.

  • Night-to-Day crash ratio is often used to decide whether the location or cluster of accidents is over represented at night.

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Generating Possible Countermeasures

  • Detailed investigations of crashes (especially crash causes)

  • Review of site plans and condition diagrams

  • Site visits

  • Other transportation engineering studies. Ex. Spot speed studies

  • The practices and previous experiences of the agency

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Economic Analysis of Countermeasures

  • Predict the effect of potential countermeasures in terms of reduced number of crashes or severity of crashes

  • Apply economic analysis (i.e. Benefit-Cost or B/C Ratio) to analyze countermeasure alternatives.

  • Narrow the range of possibilities to one or more measures to be implemented.

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Evaluation of Treatments (1)

  • Definition of Treatment:

    An intervention of some kind, a change in some design feature, or, more generally, a change in a factor that can cause a change in the safety of the unit. We also use the word “treatment” to mean “difference in attribute or feature”. That is if one unit will receive feature X and another unit feature Y, the two units are said to have received a different treatment.

  • Transportation professionals and others make decisions which affect people’s safety. Professionalism requires that the safety consequences of such decisions be known.

  • Much research on road safety revolves around the question of cause. That is, we wish to learn what change in safety will result from some treatment.

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Evaluation of Treatments (2)

In road safety, it is rare to evaluate treatments with experimentation except for new devices or concepts; knowledge has to be extracted from “observational studies”; that is, by interpreting data that can be found.

1. “Before-after” study

The change in accident history and in the attributes of these units from before treatment to after the treatment is used to estimate the change in safety that is due to the treatment.

2. “Cross Sectional Comparison” study

Two sets of sites should be identified with similar control factors except the improvement countermeasures.

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Evaluation of Treatments (3)

  • In the current engineering practice, crash reduction factors (CRFs) are often used to estimate the expected reduction in crashes that will occur during a given period as a result of implementing a proposed treatment.

    CRF=Crash # Reduced / Crash # before

  • Crash reduction factors is needed to perform an economic evaluation of a potential treatment.

  • Before-and-after study and cross-sectional methods are two existing methods for developing crash reduction factors.