Urban vs. Rural?

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

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Urban vs. Rural?

Postby prytyka » Thu Nov 08, 2012 6:35 am

I had originally posted this topic under "Human Factors and Fundamentals", but it affects the way we apply Part C - Predictive methods. I'm hoping it will generate more comment here:

The Highway Safety Manual states that “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.”

Florida has a number of facilities that exhibit a “rural” cross section (i.e. paved shoulders and roadside ditches) within urban areas as defined above. We also have a number of facilities that exhibit an “urban” typical section (i.e. curb & gutter, closed drainage system, etc.) outside of urban areas as defined above.

In conducting HSM analyses, should we assign the facility type based on its geographic location as defined above, or based on its geometric features (presence of ditches vs. curb & gutter)?
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Re: Urban vs. Rural?

Postby mdimaiuta » Fri Nov 16, 2012 11:25 am

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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