Comparison of Existing Conditions to a Proposed Condtion

Safety performance functions, development (calibration, crash modification factors [Part C], crash proportions), IHSDM and other software

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Comparison of Existing Conditions to a Proposed Condtion

Postby Joe O » Mon Oct 31, 2011 3:38 pm

Section C.7 on page C-19 discusses methods for estimating the safety effectiveness of a proposed project. Method 1 is to apply the Part C predictive method to estimate the expected average crash frequency of both the existing and proposed conditions.

If I have an existing condition and know the crash history I can apply the Part C predictive method along with the EB method to determine an expected crash frequency. For the proposed condition, assuming there is also an spf for that condition, I could apply the Part C predictive method but not EB (since no crash history exists for the proposed condition) to get a predicted crash frequency.

I'm wondering if one should still use the EB method for the existing condition but then only the spf for the proposed condition? Or would it be more "apples to apples" (and therefore more appropriate) to apply the predictive method without EB in both scenarios?

Thanks for any help.
Joe O
 
Posts: 11
Joined: Fri Jan 14, 2011 9:29 am

Re: Comparison of Existing Conditions to a Proposed Condtion

Postby mdimaiuta » Tue Nov 08, 2011 3:13 pm

Joe,

For the proposed conditions, you will first need to determine if the EB method is applicable. As noted in HSM Volume 2, Section A.2.1 (Determine Whether the EB Method is Applicable):

“If a future project is being planned, then the nature of that future project should be considered in deciding whether to apply the EB Method.

The EB Method should be applied for the analyses involving the following future project types:
• Sites at which the roadway geometrics and traffic control are not being changed (e.g., the “do-nothing” alternative);
• Projects in which the roadway cross section is modified but the basic number of through lanes remains the same (This would include, for example, projects for which lanes or shoulders were widened or the roadside was improved, but the roadway remained a rural two-lane highway);
• Projects in which minor changes in alignment are made, such as flattening individual horizontal curves while leaving most of the alignment intact;
• Projects in which a passing lane or a short four-lane section is added to a rural two-lane, two-way road to increase passing opportunities; and
• Any combination of the above improvements.

The EB Method is not applicable to the following types of improvements:
• Projects in which a new alignment is developed for a substantial proportion of the project length; and
• Intersections at which the basic number of intersection legs or type of traffic control is changed as part of a project.

If the EB Method is applied to individual roadway segments and intersections, and some roadway segments and intersections within the project limits will not be affected by the major geometric improvement, it is acceptable to apply the EB Method to those unaffected segments and intersections.”

Therefore:
• If your proposed conditions are such that the EB method is applicable, then you could use EB for both the existing and proposed conditions.
• If the EB method is applicable to only some proposed segments and intersections, then it is acceptable to apply the EB method to only those (using the Part C methods without EB for the others), as well as to the existing conditions.
• If your proposed conditions are such that the EB method is not applicable to any segment or intersection, then you can still use the EB method for the existing conditions. For the proposed, you would apply the Part C predictive method without EB.

I hope this helps.

Regards,
Mike Dimaiuta
(IHSDM.Support@dot.gov)
mdimaiuta
 
Posts: 29
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Re: Comparison of Existing Conditions to a Proposed Condtion

Postby Joe O » Wed Nov 09, 2011 12:27 pm

Hi Mike,

That does help. Specifically, your last bullet answers my question. I just wasn't sure if it was appropriate to use EB for the existing condition if it was inappropriate to use it for the proposed future condition.

Thanks for the help.

Joe
Joe O
 
Posts: 11
Joined: Fri Jan 14, 2011 9:29 am


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