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.