Roadway Hierarchies

Snelling Avenue, or MN Trunk Highway 51.

Over the past decade or so, the hierarchical roadway system used by engineers and planners to classify roadways in the US has increasingly received criticism for being outdated. Many individuals blame the hierarchy system for the predominant suburban development pattern. For example, the author(s?) of the Wikipedia entry titled Street Hierarchy don’t try very hard to hide their bias, and would have the casual reader convinced that the hierarchy system is synonymous with suburbanism.

This isn’t true, of course. Classifying roadways into a hierarchy continues to be the most logical (and historical) way to develop roadway networks (including urban grids). Hierarchies can exist within grid networks, without cul-de-sacs, with integrated land uses, with robust pedestrian and bike networks, and without forcing reliance on automobiles.

However, this doesn’t mean that our existing hierarchical systems are optimal, or that we should just continue doing exactly what we’ve been doing. There are some very clear drawbacks and challenges associated with our existing system, but I don’t think eliminating the hierarchy will solve the problems – it will only create new problems. We need to recognize the limitations of the existing hierarchies, and modify the system to address these challenges. We don’t need to eliminate the hierarchy – we need a more complex, more flexible hierarchy that does a better job accommodating the needs of urban and rural areas.

Roadways in the US are classified in at least two separate hierarchies: functional classification, and jurisdictional classifications. The two systems are roughly correlated, though not nearly as much as they should be.

Functional Classification describes the purpose and operational characteristics of a roadway and includes principal arterials (think freeways), minor arterials, collectors, and local roadways. Each of these classifications may include sub-categories.

Jurisdictional Classification describes who owns the roadway and includes state (a.k.a. Trunk Highways)(for this discussion, we can assume all federal roads are owned by the state), counties, cities/townships, and private roadways.

The following are a few of the most common scenarios that demonstrate how applying the hierarchical system on the ground becomes challenging:

Mismatched Land-use, Access, Mobility, and Hierarchical Level
This is probably the most common implementation challenge that results using the hierarchical system, particularly when applying the hierarchical system retro-actively to existing networks. This challenge occurs when the land use and urban context of the roadway are not well aligned with the functional classification. The classic example is a small town whose main street is also a busy state Trunk Highway. A roadway can not provide both the land-use and access characteristics expected of small-town main streets as well as the mobility expectations of trunk highways. As a result, these roadways often perform poorly in all respects. A more robust functional classification system could do a better job recognizing the differences in expectations of Trunk Highways in undeveloped areas from urban cores. (NOTE: The existing system does this somewhat by applying different access management requirements in urban & rural areas, but the system could be further improved.) Another good example of this problem is a collector roadway that is designed with the land-use and access characteristics of a minor arterial.

Over-reliance on a Single Level of the Hierarchy or Levels Underdeveloped
In some areas, communities become over-reliant on a single level of the hierarchy, or a particular level of the hierarchy is underdeveloped. For example, in some communities, it’s difficult to get anywhere without using a county roadway, or a minor arterial, usually because all of the local roadways have been severed to accommodate the larger roadways. This is an indication that too many roadways have been classified too high (typically, not enough collectors, too many minor arterials). If the functional classification system in a community funnels too many trips onto higher class roadways too often, the system is unbalanced and will function poorly.

Mismatched Jurisdictional and Functional Class
There is a rough correlation between jurisdictional and functional class, but not as much as there could be. For example, some county roadways are principal arterials, others function much more like collector roadways. Agencies are capable of managing a wide range of roadway types, but on occasion you find roadways that are probably managed by the wrong agency for the particular facility type. On occasion, when this disparity becomes clear to the involved agencies, the roadway will be transferred between jurisdictions. In addition, the State-Aid system that allows cities and counties to use state dollars on local or county roadways substantially muddies the water. Cities or counties own the roads, but any improvements made to that roadway must pass a MnDOT review process and must employ state design standards.

Funding Disparities between Functional Class Hierarchy Levels
The funding available for each level of the hierarchy is not equal (I’m not saying it necessarily should be). Roadways in a higher functional class are eligible for more funding sources than roadways in a lower functional class. There are very few sources of funding for local roadways other than local taxes and assessments. Federal transportation dollars are typically available only for principal and minor arterial roadways. This creates an incentive for jurisdictions to request the highest classification possible for each roadway. The result is that some roadways are classified too high.

Funding Disparities between Jurisdictional Class Hierarchy Levels
The agencies that own roadways are also subject to different funding sources. While it’s logical that each jurisdiction should be responsible for funding improvements to their own roadways, disparities often exist that can result in questionable use of funds. For example, imagine a local community has money to use for roadway reconstruction. The greatest need for investment in the community may be a county or state roadway that runs through the municipality, but there is little incentive for cities to use local funds to improve county or state roadways. In the end, the city may choose to use the local funds on projects of secondary importance on local roadways while the primary needs on a county or state roadway remain unmet. (NOTE: This being said, collaboration and funding participation between cities, counties, and the state to accomplish shared objectives on county and state roadways is commonplace and highly encouraged at all levels.)

The problems with our current hierarchical systems are clear. There is room for improvement. However, we don’t have to scrap the system entirely to improve. I believe that a clear hierarchy of roads is important for a community, and that the problems I’ve identified here can be solved through improvements to the existing hierarchical systems. The arterials and collectors we build tomorrow don’t have to look, feel, or operate like the arterials and collectors we built yesterday.


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3 Responses to Roadway Hierarchies

  1. Alex Bauman
    Alex February 8, 2012 at 3:53 am #

    Good job explaining the issue and specific problems. I think I get what you're saying on most of the scenarios, although I think some examples would have helped, particularly on Mismatched Jurisdictional and Functional Class. Are you saying that applying State Aid standards to a lower-class roadway effectively upgrades that roadway to a higher class?

    Also: Is it too hippie to suggest that part of the problem may stem from thinking of the distinctions as a hierarchy rather than merely as designing roads differently for different functions or environments? In other words, does thinking of roads as a hierarchy with principal arterials on top make engineers and policymakers strive to design all roads as principal arterials?

    • Reuben Collins
      Reuben Collins February 8, 2012 at 4:22 am #

      @Alex, I agree that specific examples would have helped. I had some examples in mind while writing, but decided against using specific examples just to avoid pointing fingers.

      That's a really good question about the hierarchy. There very well may be some of this dynamic happening.

    • Howard Q. Bikeman February 8, 2012 at 6:20 am #

      @ Alex: you make a good point about the hierarchy aspect. There is an assumption in the system that trips flow towards ever bigger roads, i.e. the Interstates. One alternate is to use "street typologies" which identify the primary purpose of the roadway: neighborhood street vs urban mixed use vs downtown core vs regional connector. That allows segments to be described by both street design as well as adjacent context and removes some aspect of "vehicle throughput" from the decision. Not for all areas, but works particularly well in compact or built-out locations.