CMF's for Urban/Suburban Arterial Intersections

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

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CMF's for Urban/Suburban Arterial Intersections

Postby Joe O » Mon Jan 24, 2011 9:16 am

The CMF's for Multi-Vehicle and Single-Vehicle Crashes at Urban/Suburban Arterial Intersections are shown on pages 12-43 through 12-45. Among other things they include CMF's for the presence of left turn lanes and right turn lanes. On page 12-39 it notes: "The CMF's used in Chapter 12 are consistent with the CMF's in Part D, although they have, in some cases, been expressed in different form to be applicable to the base conditions of the SPFs."

The SPFs for 4-legged signalized intersections are valid with AADT of 0 to 67,700 vehicles per day on the major street and 0 to 33,400 vehicles per day on the minor street (page 12-29). However, when I look in Part D the CMFs for left and right turn lanes have a much narrower range of AADT for which it says they are valid (see Tables 14-11 and 14-15). That being the case, are these CMF's still usable at locations where the volumes are outside these ranges? If not, what is the proper approach? Can you use an SPF without applying CMF's?

Along the same line, the CMF's in chapter 12 all note that they are for total accidents only. At the same time the methods for signalized intersections call for the application of SPF's for fatal/injury accidents as well. What is the proper way to do this without valid CMF's for fatal/injury accidents?
Joe O
 
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Re: CMF's for Urban/Suburban Arterial Intersections

Postby mdimaiuta » Fri Jan 28, 2011 5:02 pm

Joe - my response is below, in red:

"The CMF's for Multi-Vehicle and Single-Vehicle Crashes at Urban/Suburban Arterial Intersections are shown on pages 12-43 through 12-45. Among other things they include CMF's for the presence of left turn lanes and right turn lanes. On page 12-39 it notes: "The CMF's used in Chapter 12 are consistent with the CMF's in Part D, although they have, in some cases, been expressed in different form to be applicable to the base conditions of the SPFs."

The SPFs for 4-legged signalized intersections are valid with AADT of 0 to 67,700 vehicles per day on the major street and 0 to 33,400 vehicles per day on the minor street (page 12-29). However, when I look in Part D the CMFs for left and right turn lanes have a much narrower range of AADT for which it says they are valid (see Tables 14-11 and 14-15). That being the case, are these CMF's still usable at locations where the volumes are outside these ranges? If not, what is the proper approach? Can you use an SPF without applying CMF's?

If you use an SPF without applying CMFs, then the result will be expected crashes on a facility similar to those used to develop the SPF, with the AADTs that you entered, and using "base conditions" (all CMFs = 1.0) for geometric elements and other conditions reflected in the CMFs. CMFs are required to account for the actual geometrics and conditions of the site that you are evaluating.

Along the same line, the CMF's in chapter 12 all note that they are for total accidents only. At the same time the methods for signalized intersections call for the application of SPF's for fatal/injury accidents as well. What is the proper way to do this without valid CMF's for fatal/injury accidents?"

We will respond to this issue in a separate post.
mdimaiuta
 
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Re: CMF's for Urban/Suburban Arterial Intersections

Postby Joe O » Mon Jan 31, 2011 9:46 am

Thanks for the response. I appreciate the information. Based on your response, it is important to use CMF's to adjust for the base conditions for which the SPF's were developed. That brings me back to the first part of my question. Some of the CMF's have a fairly narrow traffic volume range for which they are applicable (for example, see left and right turn lanes in tables 14-11 and 14-15). Many of my 4-legged signalized intersections fall outside the volume ranges noted for the CMF's. It seems that either we have to apply the CMF's outside the noted volume ranges or . . . the methodology becomes fairly limited if we are bound by those volume ranges. Am I missing something?

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

Re: CMF's for Urban/Suburban Arterial Intersections

Postby mdimaiuta » Tue Feb 01, 2011 12:32 pm

Joe - my reply is in red:

"Some of the CMF's have a fairly narrow traffic volume range for which they are applicable (for example, see left and right turn lanes in tables 14-11 and 14-15). Many of my 4-legged signalized intersections fall outside the volume ranges noted for the CMF's. It seems that either we have to apply the CMF's outside the noted volume ranges or . . . the methodology becomes fairly limited if we are bound by those volume ranges. Am I missing something?"

In Part C, an implicit assumption seems to be that the CMFs can be applied to the same range of AADTs as the SPFs; although - as you noted - some CMFs in Part D (e.g., left- and right-turn lanes in Chapter 14) show a different range of AADT, based on the data used to develop the CMFs.
With no CMFs for AADTs outside the ranges specified in Ch. 14, our recommendation is to use the Part C methods as presented, i.e., use the CMFs, as long as your AADTs do not deviate substantially from the range suggested for the SPFs. If your AADT is outside the range suggested for an SPF, understand that “application to sites with AADTs substantially outside these ranges may not provide reliable results” (HSM, p. 12-18 and p. 12-29). Of course, the term “substantially” is subjective and requires engineering judgment…

We will contact the model developer for further clarification on this issue.


Regarding the other part of your post -- "...the CMF's in chapter 12 all note that they are for total accidents only. At the same time the methods for signalized intersections call for the application of SPF's for fatal/injury accidents as well. What is the proper way to do this without valid CMF's for fatal/injury accidents?"

Lacking guidance in Part C regarding this issue, we recommend assuming that the CMFs for total crashes also apply to fatal/injury crashes. In other words, assume that the effect on fatal/injury crashes is the same as the effect on total crashes.

Some CMFs in Chapter 12 do reference fatal/injury crashes vs. total crashes. For example, for segments, CMF5r (Automated Speed Enforcement) has a value of 0.95 for total crashes, while the text notes that “Chapter 17 presents a CMF of 0.83 for the reduction of all types of fatal-and-injury crashes…” (HSM, p. 12-43, CMF5r).

Again, we will follow-up with the model developer for further clarification on this issue.
mdimaiuta
 
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Joined: Tue Nov 30, 2010 1:50 pm


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