Calibration Model/Prioritization

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

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Calibration Model/Prioritization

Postby andrew.malizia » Thu Jun 09, 2011 10:12 am

Hello all,
I am working on calibrating a rural intersection model for my agency, Stanislaus County. I have gone through and recreated the HSM equations in VBA in Microsoft excel to adjust actual vs HSM predicted collisions.

One of the issues I face is that the predictive method only is for minor stop-controlled intersections and the SPF's are limited. I've built my predictive model using CMF's to alter the predicted collisions at EXISTING intersections so that I could use teh All-way stop CMF as well as a few others. Is this an acceptable use of the methods in the HSM?

For instance, my spreadsheet uses these factors:
Major ADT, Minor ADT, Urban/Rural, 2lane/Multi, Type (3st/4st/4sg), All-way (Y/N), Stop ahead (y/n), flashing beacons (y/n), lighting (y/n), Skew Angle, left turn lanes, right turn lanes.

As you can see I've added all-way, stop ahead, and flashing beacons to my predictive model. I can't see why these CMF's wouldn't work in the same was an an SPF. The All-way stop was the biggest reason for adding the CMF's to the model. Has anyone done anything similar?

I would be happy to share my spreadsheet once I have it where I want it to be. Right now it only works for Rural/2Lane intersections. But I plan on adding Urban and multi lane in the future.

Thanks,
Andrew Malizia, PE
Stanislaus County PW
andrew.malizia
 
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Joined: Tue Jun 07, 2011 10:11 am
Location: Modesto, CA

Re: Calibration Model/Prioritization

Postby mdimaiuta » Mon Jun 13, 2011 12:59 pm

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
mdimaiuta
 
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Joined: Tue Nov 30, 2010 1:50 pm

Re: Calibration Model/Prioritization

Postby andrew.malizia » Mon Jun 13, 2011 4:47 pm

Thanks for all the great info Mike. The reason for reproducing the software was because I was able to actively compare and create a calibration model more effectively than I could in the IHSDM-HSM software. I'm looking at my calibration model, not only as the calibration model, but as a top down approach at locating potential safety projects. I am comparing actual collision history to the predictive method, but only to compare with 'like facilities' and determine which intersections have a collision rate that exceeds the predictive methods return. I'm not entirely concerned about how many PDO, Injury, fatals, night time, etc... at the top down level. It will be through further use of the IHSDM_HSM software that all of that is calculated using the base model as a solution to find 'potential projects." It's far easier to compare 10 intersections at a time, then 100.

I am not attempting to mis-use or misrepresent the HSM in anyway and that is why I came to this forum. I am very much interested in using it as an effective safety tool, but I really needed that step 1 to identify potential locations. I am ranking them based on the difference in actual collision history vs. predictive method. Using a factor of actual/predicted seemed to misrepresent a little as .100 collisions per year predicted vs 1 actual would be a factor of 10, where say predicted in another location is 2 collisions per year, but actual is 6 leaving a factor of only 3. Being that time is of the essence in safety projects, we can look at it quantitatively and say that this location has FOUR more collisions than expected where the first location only had .9/yr and therefore the reduction of 4 is "more effective."

Is there a way in the IHSDM to actually compare various locations in a way that I am doing? My biggest concern was coming up with the calibration factor for 2lane, 2way rural intersections, then multilane, and eventually urban. I fully intend to continue this process for segments as well.
Andrew Malizia, PE
Stanislaus County Public Works
1716 Morgan Rd
Modesto, CA 95358
andrew.malizia
 
Posts: 2
Joined: Tue Jun 07, 2011 10:11 am
Location: Modesto, CA


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