original data used in HSM SPF development

Data needs and management, crash severity levels and human factors

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original data used in HSM SPF development

Postby godfrmar » Wed Oct 24, 2012 12:12 pm

Was the oringinal data that was used to develop the HSM SPFs consider the facility type, rural or urban, based on FHWA's definitions?

(urban = places inside urban boundaries where the population is greater than 5,000 persons; rural = places outside urban areas which have a population less than 5,000 persons)
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Re: original data used in HSM SPF development

Postby mdimaiuta » Fri Nov 16, 2012 11:37 am

I assume that the data used to develop the Safety Performance Functions (SPFs) considered the facility type based on FHWA definitions. A similar issue was posted in the Part C section of the forum, focused on classifying roadways for evalution with the Part C predictive methods rather than the data used to develop the SPFs. My response to that post:

The HSM seems to allow for user discretion in this case, though it is not entirely clear. On p. 12-2, it states, “Classifying an area as urban, suburban, or rural is subject to the roadway characteristics, surrounding population and land uses and is at the user’s discretion.” However, as noted in your post, it then continues, “In the HSM, the definition of ‘urban’ and ‘rural’ areas is based on Federal Highway Administration (FHWA) guidelines which classify ‘urban’ areas as places inside urban boundaries where the population is greater than 5,000 persons. ‘Rural’ areas are defined as places outside urban areas which have a population less than 5,000 persons. The HSM uses the term ‘suburban’ to refer to outlying portions of an urban area; the predictive method does not distinguish between urban and suburban portions of a developed area. The term ‘arterial’ refers to facilities that meet the FHWA definition of ‘roads serving major traffic movements (high-speed, high volume) for travel between major points.’”

The issue that you raise also came up a few years ago. A State DOT was trying to calibrate the prediction models and had a number of roadway sections in what FHWA would classify as rural areas, but whose characteristics (sidewalks, curb & gutter, parking, etc.) and surrounding land use (small town) suggested that they functioned more like urban roads. At the time, the developer of the predictive methods in HSM Chapters 10 and 12 recommended that HSM users use their judgment to include such roads in one category or another (i.e., rural or urban/suburban).

So, as a minimum, it is certainly acceptable to assign the facility type based on a roadway’s geometric features and surrounding land use. You should not feel constrained by the FHWA guidelines if other indicators strongly suggest otherwise.

Mike Dimaiuta
IHSDM / HSM Support
ihsdm.support@dot.gov
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