Road classification update

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Re: Road classification update

Postby davielde » Tue Nov 19, 2013 6:21 am

The document is a snoozer in general, but here is a detailed explanation of how the FHWA suggests roads be classified functionally. Skip to tables 3-5 and 3-6 on pages 22 and 23 of the memo (pages 28 and 29 of the PDF) for the summary.

http://www.michigan.gov/documents/mdot/ ... 6823_7.pdf

The Federal Highway Administration provides a breakdown of guidelines for individual states to adopt as they see fit, and the state classifications are then sent back to the FHWA for approval. There are a lot of uses of "in general" and "typically" when describing how factors such as number of access points, mileage extent, Vehicle Miles of Travel (VMT), Annual Average Daily Traffic (AADT), route spacing, speed limit, efficiency of travel, etc. come into play. This unfortunately translates to a lot of grey area in the interpretation, which MDOT apparently had no qualms about taking advantage of when it came to classifying arterials in the Detroit area.

The NFC uses a percentage model where a wide-ranging percent of each road type needs representation, and this varies from a rural to urban setting. Efficiency is the key consideration. For an urban "other principal arterial", efficiency in the ability to route over a longer distance is actually more important than traffic volume stats and other factors. The summary describes it as a "high proportion of total urban travel on minimum of mileage", but volume (AADT) is ultimately a result of greater efficiency, not the cause. Given two identical roads and the need to classify one as a principal and the other as minor though, the guidelines state that AADT would be the "tie-breaker". In addition, the number of access points are "typically" based on the desired classification, and speed limit is "typically" higher as a result of fewer at-grade connections.

With that said, there are general rules laid out in tables 3-5 for arterial classification. Urban principal arterials should make up 4%-5% of the physical miles but carry 16%-31% of the traffic, and urban minor arterials should make up 7%-14% of the physical miles and carry 14%-25% of the traffic. AADT volume can range from 7k to 27k for an urban principal arterial whereas an urban minor arterial can range from 3k to 14k. Notice that nice overlap from 7k at the low range of principal to 14k at the high range of minor. It's almost of if staff at the FHWA realized they needed to put something out there, but they didn't really care since the states would be doing all the work according to their local understanding... This setup unfortunately leads to a lot of standardization within the state but not necessarily much from state to state. To continue to illustrate how flexible this is, access can be partially controlled for a principal, or it could be completely uncontrolled just like a minor. It could be either divided or undivided. Interestingly, lane width, shoulder width, and access are taken into consideration as guidelines for arterials; but items such as the number of lanes, speed limit, etc. are "typically" the result of the classification as work is done to make the route more efficient over time. Maybe that's why they added those roundabouts to Maple Rd and 14 Mile in Farmington, both of which are classified as principal arterials for those stretches...

At the end of the day though, the point is that there is a lot of overlap and room for uncertainty. MDOT made a judgment for Michigan based on these vague guidelines, but at least the result is able to be translated easily to a map. Hopefully this at least provides a better understanding of the "how" and "why" of certain classifications in a lot of cases where they may not otherwise be intuitive.
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Re: Road classification update

Postby Riamus » Tue Nov 19, 2013 1:58 pm

Very nice posts, davielde. And thank you for linking the document explaining the classifications. I didn't read the entire thing of course, but I did scan through the classifications themselves and a few other areas and it's quite informative. As you said, some classifications may just feel "off," but there is likely a good reason for that classification that we just are not aware of. And for everyone, don't forget that we can still increase the classifications if we feel that there is a good reason to do so. I did that for a short little section of road (probably under 100ft) because the NFC had a major collector running east toward a state highway, stop just short of it and head south. It just really made sense to connect the collector to the state highway instead of leaving it as a Street. So the NFC is the main guideline, but it can be tweaked if necessary. You should avoid downgrading a road from what the NFC says unless there is a good reason, however. For example, if a Major Collector gets permanently closed on one end and you are certain that it will no longer be a Major Collector once the NFC is updated.
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Re: Road classification update

Postby Riamus » Wed Nov 20, 2013 5:21 pm

Note that the first post in this thread has been updated with what I've completed.
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Re: Road classification update

Postby davielde » Wed Dec 11, 2013 8:59 pm

It looks like the Michigan NFC maps have been updated today to "v13". The urban/rural distinction is now clearer, but the colors changed, so be careful. Particularly with regard to the brown "minor arterial" versus grey local roads, I find it almost impossible to distinguish unless highly zoomed in. Green has now changed to minor collector instead of minor arterial. On a good note, the higher available resolution when zoomed in is a bonus.
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Re: Road classification update

Postby sketch » Wed Dec 11, 2013 9:25 pm

davielde wrote:It looks like the Michigan NFC maps have been updated today to "v13". The urban/rural distinction is now clearer, but the colors changed, so be careful. Particularly with regard to the brown "minor arterial" versus grey local roads, I find it almost impossible to distinguish unless highly zoomed in. Green has now changed to minor collector instead of minor arterial. On a good note, the higher available resolution when zoomed in is a bonus.

That's irritating. I had been using the urban/urbanized area maps from the dropdown, rather than the county maps, because they didn't include local roads, so they were easier to read and didn't bog my system down so badly.
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Re: Road classification update

Postby miked_64 » Thu Dec 12, 2013 5:38 pm

So now do the colors/descriptions map as this?

NFCWaze Map Editor
Blue - InterstateBlue - Freeway
Purple - FreewayBlue - Freeway
Red - Principle ArterialRed - Major Highway
Brown - Minor ArterialYellow - Minor Highway
Orange - Major CollectorOrange - Primary Street
Green - Minor CollectorOrange - Primary Street
Grey – LocalWhite – Street
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Re: Road classification update

Postby sketch » Thu Dec 12, 2013 5:56 pm

Looks like it, yep. The federal standards say "other freeway or expressway" for the "purple" class, but glancing at the MDOT maps, it appears they have chosen not to use this class for partially-limited-access "expressways" like M-53 between 27 and 34.
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Re: Road classification update

Postby keeegs » Thu Dec 19, 2013 4:06 am

miked_64 wrote:So now do the colors/descriptions map as this?

NFCWaze Map Editor
Blue - InterstateBlue - Freeway
Purple - FreewayBlue - Freeway
Red - Principle ArterialRed - Major Highway
Brown - Minor ArterialYellow - Minor Highway
Orange - Major CollectorOrange - Primary Street
Green - Minor CollectorOrange - Primary Street
Grey – LocalWhite – Street


Be careful, colors for classifications can change between counties.

Kent county for example switches the colors between Minor Arterial and Minor Collector.
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Re: Road classification update

Postby miked_64 » Thu Dec 19, 2013 5:26 pm

Captnkeegs - It looks to me like the v13 Kent Co. map follows this color scheme as well. Is it possible that you have an old link?

http://mdotcf.state.mi.us/public/maps_nfc/pdf/NFC13_KENT.pdf
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Re: Road classification update

Postby Boiler_81 » Mon Dec 30, 2013 7:07 pm

I think it would be helpful if we coordinated editing the Detroit metro area. If people would list in this thread the areas they have edited and are working on it would help us from bumping into one another and analyzing areas which have already been done.
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