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Crash Modification Factors in Practice: Quantifying Safety in the Development and Analysis of Alternatives - Arizona and Colorado Case Studies

Summary from Crash Modification Factors in Practice: Using CMFs to Quantify Safety in the Development and Analysis of Alternatives

(The case studies begins on Page 17 of the full report, after background information about the use of crash modification development and analysis of roadway safety alternatives.)

Publication Year: 2013


Background

Crash Modification Factors (CMFs) can be applied in the development and analysis of alternatives to estimate the safety performance when the advantages and disadvantages of each alternative are considered. The following case studies illustrate how CMFs have been applied by the Colorado Department of Transportation (CDOT) and the Arizona Department of Transportation (ADOT) in the development and analysis of alternatives.

Case Study #1: Colorado

The following case study illustrates how the Observed Crash Frequency with CMF Adjustment method has been used to assess the safety impact of alternatives. Information for the case study was provided by CDOT.

Project Description

Castle Rock, Colorado lies south of Denver along the Interstate 25 corridor. To accommodate growing development in the area, CDOT considered a new interchange on I-25. In addition to the “no build” scenario, they considered two alternatives for the new interchange design. Alternative 1 would extend one road, Castlegate Drive, to create the new interchange. Alternative 2 would extend another road, Atrium Drive, to create the new interchange.

As part of the environmental assessment of the project in 2009, CDOT conducted a safety analysis to evaluate the effect on crashes for the proximate roadway segments and intersections, including ramp junctions. The full safety analysis developed estimates of crash predictions for each segment and junction based on either Safety Performance Functions (SPFs) (for segments) or comparisons to similar intersections in the area (for intersections). At the time of the analysis, CDOT did not have available SPFs for intersections.

Findings

This case study presented an example of how CMFs can be applied to estimate the safety impacts of various alternatives. The safety analysis presented in this case study was just one piece of the overall safety analysis conducted for the proposed interchange alternatives. In addition to the safety analysis of the alternative junction types, CDOT developed crash estimates for each segment and intersection within the study area. The result was an estimate of annual crashes for the entire study area for Alternatives 1 and 2. The estimated safety performance of each alternative can then be considered with the operational performance, project costs, environmental impacts, and other factors to identify a balanced design and the most desirable alternative.


Case Study #2: Arizona

The following case study illustrates how the Expected Crash Frequency with CMF Adjustment method has been used to quantify the safety impacts during the development and analysis of alternatives. Information for the case study was provided by ADOT.

ADOT is performing predictive analyses following the procedures in the AASHTO Highway Safety Manual at the scoping and alternative selection stage of demonstration projects. They are working to develop a framework for integrating substantive safety considerations into the ADOT project planning and development process.

Project Description

ADOT identified potential safety improvements on a 24.6 mile section of Arizona State Route 264 (SR 264) and evaluated the potential safety impacts during the analysis phase of the development and analysis of the alternatives. SR 264 is a rural, two-lane road in northeastern Arizona and functionally classified as a minor arterial. Figure 2 identifies the general location and limits of the study section.

Findings

This case study presented an example of how the Expected Crash Frequency with CMF Adjustment method can be used to estimate the expected safety impacts of various design alternatives. ADOT used SPFs and CMFs from the Highway Safety Manual (HSM) in this analysis, supported by the Interactive Highway Safety Design Model (IHSDM) software. They also incorporated observed crash history, using the Empirical Bayes method, to estimate the expected crashes for various scenarios. The result was an estimate of total expected crashes for the entire study section over a 20-year analysis period. This allowed for a quantitative comparison of the safety performance for two design alternatives and the existing conditions. ADOT used the results of the crash analysis in a benefit-cost analysis to help select the most cost-effective alternative.

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Contact

Karen Scurry
FHWA Office
609-637-4207
Karen.Scurry@dot.gov

Publication Year: 2013

View more information about CMFs on the web at: http://safety.fhwa.dot.gov/.
Additional practices in the CMFs in Practice Series:

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