Hello Andrew,

HSM Part C, Section C.7 (Methods for Estimating the Safety Effectiveness of a Proposed Project) describes four methods for estimating the change in expected crash frequency of a proposed project or design alternative, in order of predictive reliability:

“The Part C predictive method provides a structured methodology to estimate the expected average crash frequency where geometric design and traffic control features are specified. There are four methods for estimating the change in expected average crash frequency of a proposed project or project design alternative (i.e., the effectiveness of the proposed changes in terms of crash reduction). In order of predictive reliability (high to low) these are:

■Method 1—Apply the Part C predictive method to estimate the expected average crash frequency of both the existing and proposed conditions.

■Method 2—Apply the Part C predictive method to estimate the expected average crash frequency of the existing condition and apply an appropriate project CMF from Part D (i.e., a CMF that represents a project which changes the character of a site) to estimate the safety performance of the proposed condition.

■Method 3—If the Part C predictive method is not available, but a Safety Performance Function (SPF) applicable to the existing roadway condition is available (i.e., an SPF developed for a facility type that is not included in Part C of the HSM), use that SPF to estimate the expected average crash frequency of the existing condition. Apply an appropriate project CMF from Part D to estimate the expected average crash frequency of the proposed condition. A locally-derived project CMF can also be used in Method 3.

■Method 4—Use observed crash frequency to estimate the expected average crash frequency of the existing condition and apply an appropriate project CMF from Part D to the estimated expected average crash frequency of the existing condition to obtain the estimated expected average crash frequency for the proposed condition.

In all four of the above methods, the difference in estimated expected average crash frequency between the existing and proposed conditions/projects is used as the project effectiveness estimate.”

So, the most desirable method (Method 1) is to apply Part C methods. However, since there are no Part C models for all-way stop controlled intersections, the HSM states that Method 2 is the best alternative for your case – which is essentially what you have done (with the minor road stop controlled intersection as the “existing condition” and the all-way stop as the “proposed condition”). Note that the CMF in Part D for converting minor-road stop control to all-way stop control (Table 14- 5, p. 14-12) only applies when MUTCD warrants are met.

The Introduction to Part C also provides a discussion on applying multiple CMFs and the assumption of independent effects, which should be kept in mind:

“The CMFs are multiplicative because the most reasonable assumption based on current knowledge is to assume independence of the effects of the features they represent. Little research exists regarding the independence of these effects. The use of observed crash data in the EB Method (see Section C.6.6 and Appendix A to Part C) can help to compensate for any bias which may be caused by lack of independence of the CMFs.” (p. C-16)

“Where multiple treatments or countermeasures will be applied concurrently and are presumed to have independent effects, the CMFs for the combined treatments are multiplicative. As discussed above, limited research exists regarding the independence of the effects of individual treatments from one another. However, in the case of proposed treatments that have not yet been implemented, there are no observed crash data for the future condition to provide any compensation for overestimating forecast effectiveness of multiple treatments. Thus, engineering judgment is required to assess the interrelationships and independence for multiple treatments at a site.

The limited understanding of interrelationships among various treatments requires consideration, especially when several CMFs are being multiplied. It is possible to overestimate the combined effect of multiple treatments when it is expected that more than one of the treatments may affect the same type of crash. The implementation of wider lanes and shoulders along a corridor is an example of a combined treatment where the independence of the individual treatments is unclear because both treatments are expected to reduce the same crash types. When implementing potentially interdependent treatments, users should exercise engineering judgment to assess the interrelationship and/or independence of individual elements or treatments being considered for implementation within the same project.” (p. C-16)

Finally, you mentioned that you have recreated HSM equations in VBA in Excel. Please note that the Crash Prediction Module (CPM) component of FHWA’s Interactive Highway Safety Design Model (IHSDM) is a faithful software implementation of the HSM Part C predictive methods. The software is available free-of-charge at and my office (the FHWA Geometric Design Lab) provides free technical support (e-mail:

ihsdm.support@dot.gov; phone: 202-493-3407).

Regards,

Mike Dimaiuta