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Home > Tools and Technology > Crash Reduction Factors

Toolbox of Countermeasures andTheir Potential Effectiveness for Intersection Crashes

Introduction

This issue brief documents estimates of the crash reduction that might be expected if a specific countermeasure or group of countermeasures is implemented with respect to intersection crashes. The crash reduction estimates are presented as Crash Reduction Factors (CRFs).

Traffic engineers and other transportation professionals can use the information contained in this issue brief when asking the following types of question: Which countermeasures might be considered at the signalized intersection of Maple and Elm streets, an intersection experiencing a high number of total crashes and left-turn crashes? What change in the number of total crashes and left-turn crashes can be expected with the implementation of the various countermeasures?

Crash Reduction Factors

A CRF is the percentage crash reduction that might be expected after implementing a given countermeasure. In some cases, the CRF is negative, i.e. the implementation of a countermeasure is expected to lead to a percentage increase in crashes.

One CRF estimate is provided for each countermeasure. Where multiple CRF estimates were available from the literature, selection criteria were used to choose which CRFs to include in the issue brief:

• Firstly, CRFs from studies that took into account regression to the mean and changes in traffic volume were preferred over studies that did not.
• Secondly, CRFs from studies that provided additional information about the conditions under which the countermeasure was applied (e.g. road type, area type) were preferred over studies that did not.

Where these criteria could not be met, a CRF may still be provided. In these cases, it is recognized that the reliability of the estimate of the CRF is low, but the estimate is the best available at this time. The CRFs in this issue brief may be periodically updated as new information becomes available.

The Desktop Reference for Countermeasures lists all of the CRFs included in this issue brief, and adds many other CRFs available in the literature. A few CRFs found in the literature were not included in the Desktop Reference. These CRFs were considered to have too large a range or too large a standard error to be meaningful, or the original research did not provide sufficient detail for the CRF to be useful.

A CRF should be regarded as a generic estimate of the effectiveness of a countermeasure. The estimate is a useful guide, but it remains necessary to apply engineering judgment and to consider site-specific environmental, traffic volume, traffic mix, geometric, and operational conditions which will affect the safety impact of a countermeasure. The user must ensure that a countermeasure applies to the particular conditions being considered. The reader is also encouraged to obtain and review the original source documents for more detailed information, and to search databases such as the National Transportation Library (ntlsearch.bts.gov) for information that becomes available after the publication of this issue brief.

Presentation of the Crash Reduction Factors

In the Tables presented in this issue brief, the crash reduction estimates are provided in the following format:

CRF(standard error)^REF

The CRF is the value selected from the literature.

The standard error is given where available. The standard error is the standard deviation of the error in the estimate of the CRF. The true value of the CRF is unknown. The standard error provides a measure of the precision of the estimate of the true value of the CRF. A relatively small standard error indicates that a CRF is relatively precisely known. A relatively large standard error indicates that a CRF is not precisely known. The standard error may be used to estimate a confidence interval of the true value of the CRF. (An example of a confidence interval calculation is given below.)

The REF is the reference number for the source information.

As an example, the CRF for the countermeasure install cameras to detect red-light running for right-angle fatal/injury crashes is:

16(6)²⁷

The following points should be noted:

• The CRF of 16 means that a 16% reduction in right-angle fatal/injury crashes is expected after the installation of red-light running cameras.
• This CRF is bolded which means that a) a rigorous study methodology was used to estimate the CRF, and b) the standard error is relatively small. A CRF which is not bolded indicates that a less rigorous methodology (e.g. a simple before-after study) was used to estimate the CRF and/or the standard error is large compared with the CRF.
• The standard error for this CRF is 6. Using the standard error, it is possible to calculate the 95% confidence interval for the potential crash reduction that might be achieved by implementing the countermeasure. The 95% confidence interval is ±2 standard errors from the CRF. Therefore, the 95% confidence interval for the installation of red-light running cameras for right-angle fatal/injury crashes is between 4% and 28% (16 - 2×6 = 4%, and 16 + 2×6 = 28%).
• The reference number is 27 (Persaud et al., as listed in the References at the end of this issue brief).

Using the Tables

The CRFs for intersection crashes are presented in three tables which summarize the available information. The Tables are:

• Table 1: Signalization Countermeasures, which includes signal operations countermeasures, signal hardware countermeasures, and combination signal and other countermeasures
• Table 2: Geometric Countermeasures, which includes left turn countermeasures, right turn countermeasures, and other geometric countermeasures
• Table 3: Signs/Markings/Operational Countermeasures, which includes signs, pavement markings modifications, regulatory, lighting, and operational countermeasures

Readers familiar with the previous edition of this issue brief will notice the following changes:

• Countermeasure cost estimates of low, medium, high are no longer provided as most agencies have readily available cost estimate information with actual dollar amounts.
• Countermeasures that do not have an estimate of crash reduction effectiveness are no longer included.

The following points should be noted:

• Where available, separate CRFs are provided for different crash severities. The crash severities are: all, fatal/injury, fatal, injury, or property damage only (PDO).
• Where available, existing traffic control information is provided (i.e. the conditions existing before implementation of a countermeasure). The control information may be no signal, signal, stop, or stop/ yield. “Undefined” is used when a publication does not provide more specific information such as no signal, signal, stop, or yield controlled.
• Where available, the Tables provide daily traffic volume(vehicles/day) information for the major and minor roads of the intersection where the potential effectiveness of the countermeasure was measured. Where only one volume is provided, this volume refers to the traffic volume on the major road, unless otherwise specified.
• Blank cells mean that no information is reported in the source document.
• For additional information, please visit the FHWA Office of Safety website (safety.fhwa.dot.gov).

Table 1: Signalization Countermeasures

Table 2: Geometric Countermeasures

Table 3: Signs/Markings/Operational Countermeasures

References

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    Updated August 2008

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