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Crash Modification Factors in Practice: Using CMFs to Quantify the Safety Performance of Design Decisions and Exceptions - California and Missouri

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 18 of the full report, after background information about the use of crash modification to quantify the safety performance of design decisions and exceptions.)

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 California Department of Transportation (Caltrans) and the Missouri Department of Transportation (MoDOT) in the development and analysis of alternatives.

Case Study #1: California

The following case study illustrates how the Observed Crash Frequency with CMF Adjustment method has been used to assess the safety impact of individual design elements and evaluate the overall impact of design exceptions on the safety performance of a facility. Information for the case study was provided by Caltrans.

Project Description

In response to 24 collisions that occurred in a three-year period within a section of US 199 in Northern California, District 1 of Caltrans proposed a series of engineering improvements to address potential safety issues. The project limits are within United States Forest Service Lands in Del Norte County, approximately two miles north of Hiouchi. The limits extend from 0.9 to 1.1 miles north of South Fork Road. The existing alignment consists of two curves with a short tangent transition, forming a reverse curve. Curve 2 was the primary focus of the engineering improvements as all 24 crashes occurred along this curve during the three-year period.

Findings

CMFs can be applied to quantify the safety impacts of design elements and estimate the effects of mitigation measures. Combined, these results can be used to evaluate the overall impacts of design exceptions on the estimated safety performance of a facility. In this case, District 1 of Caltrans used CMFs in order to quantify the safety impacts of increasing the radius of a curve, increasing the superelevation, increasing the width of the travel lane, and increasing the shoulder width. Even though some of the proposed changes did not meet the design standard based on California's design documents, the use of CMFs demonstrated that the proposed improvements could result in a substantial reduction in crashes compared to the existing conditions. Further analysis could compare the estimated safety impact of proposed design exceptions with respect to design standards. The results of the safety analysis could also be considered in conjunction with other factors such as project cost, operational performance, and environmental impacts.


Case Study #2: Missouri

The following case study illustrates how the Predicted Crash Frequency with CMF Adjustment method has been used to assess the safety impact of individual design elements and evaluate the overall impact of design exceptions on the safety performance of a facility. Information for the case study was provided by the Missouri Department of Transportation (MoDOT).

Project Description

MoDOT Central District proposed a project on a rural, two-lane section of Route 42 in Kaiser, MO. The existing conditions included a narrow cross-section with lane widths of 10.5 feet and unpaved shoulders. The proposed conditions included paved shoulders (2 feet in both directions) and shoulder and centerline rumble stripes. The design guidelines for minor roads in Missouri identify minimum expectations for several design features, including a consistent shoulder width of 2 to 4 feet. In this case, the District conducted an analysis, using Part C Predictive Methods of the HSM, to document the potential safety benefits of the proposed conditions compared to the existing conditions. A separate analysis is also provided to compare the safety performance of different shoulder widths (2 feet versus 4 feet).

Findings

SPFs can be used to predict crashes for baseline conditions and CMFs can be applied to adjust the baseline estimate to reflect specific conditions of interest. This is useful for quantifying and comparing the safety performance of scenarios with different design features and can aid in the decision-making process. Specifically, this approach can help an agency to better understand the potential safety impacts of individual design elements and design exceptions. MoDOT conducts similar safety analyses as part of the evaluation of design exceptions that involve safety related features. In this case, Central District of MoDOT used the Predicted Crash Frequency with CMF Adjustment method in order to quantify the safety impacts of installing a paved shoulder with shoulder and centerline rumble stripes. Two different scenarios are compared to the existing conditions. The proposed condition included a paved shoulder width of two feet, while the alternative condition based on design guidelines is a paved shoulder width of four feet. The use of this quantitative method demonstrated that the proposed improvements could result in a substantial reduction in crashes compared to existing conditions. Recall that non-calibrated SPFs may overestimate or underestimate the predicted crash frequency, but provide a reasonable estimate of the percent difference in crashes among alternatives. As such, it is desirable to use a calibrated SPF if it is necessary to estimate the change in predicted crash frequency or conduct a formal economic analysis.

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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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