The Hierarchy of Roads: 7 Axioms on street design

I posit several Axioms about the hierarchy of roads.

Axiom 1: Some roads should be fast

The aim of transport is connecting people with destinations. They can connect with more destinations if they can do so in less time. Ceteris paribus, faster roads will take less time. Without loss of generality, let’s call these roads highways.

Axiom 2: Some roads should be slow

Some roads serve neighborhoods and have traffic that is not just motor vehicles. Ceteris paribus, slower roads are more likely to ensure safety [both reducing the probability of a collision through higher reaction times and reducing the impact of a collision should one occur], a high quality of life, and increased interaction within the neighborhood. Without loss of generality, let’s call these roads streets.

Axiom 3: Fast roads (highways) attract traffic from slow roads (streets)

In general, people prefer to spend less time traveling, and will spend less time on faster roads. These roads will attract more people. There will be net reductions in traffic on streets that are made slower and net increases in traffic on roads that are made faster.

Axiom 4: Urban design, congestion, safety, and funding problems arise when streets and highways are confused.

People, who are soft and move slowly, do not mix with vehicles, which are hard, when they move fast. If people feel unsafe they will avoid the place. Streets functioning as highways and managed by higher levels of government will be redesigned to be highways, — what Charles Marohn of Strong Towns calls “stroads” — destroying their street function.

Streets types matrix from Transport for London looking at tradeoff of Movement and Place. http://content.tfl.gov.uk/street-types-matrix.pdf
Streets types matrix from Transport for London looking at tradeoff of Movement and Place.

Further trying to move highway levels of through traffic on roads initially designed as streets with lots of access and at grade intersections is a natural misfit that will result in local congestion. At least this limits the amount of through traffic. Traffic and demand comprise a negative feedback system, more traffic slows speeds –> slower speed lowers demand –> less demand reduces traffic.

Axiom 5: Without strict controls, properties will try to gain direct access to highways.

Many streets started out as highways in previous generations with earlier technologies.  They were once crossroads that attracted businesses and became a place. This is the dual or mirror of the “Stroads” problem, in analogy, we might call them “Reets“.

While this origin story is not of itself a problem, the road should be designed for what it does, and what we want it to do, not what it once did. Highways with traffic are attractive places to open businesses. The US Highway System (the national system before the Interstate, which still exists) was plagued with this problem, once freeflowing roads were subject to steady speed deterioration as new motels, gas stations, restaurants, and stands emerged to exploit the traffic. By design to overcome this problem, the Interstate was more regulated in this regard, and was instead a limited access facility.

Axiom 6: Successful streets will attract more traffic.

Streets that have lots of local activity will encourage vehicle traffic as people seek to take advantage of the activity, and park their vehicle nearby. This does not justify “upgrading” the street through widening, which takes out the very elements that made it successful in the first place.

Axiom 7: The Hierarchy of Roads is an emergent process.

Even in the absence of central planning, a hierarchy of roads would emerge. Some roads will become more important than others just because of randomness, geography and topology, and positive feedback effects. Local roads naturally serve more local traffic, and we can distinguish the importance of roads by the source of their users. While A City is Not a Tree as Christopher Alexander said, it does have hierarchical features.


New automated vehicles can be better regulated than mere humans. There will also be a new Cambrian explosion of vehicle forms which are specialized for markets, especially in urban areas where mobility as a service is plausible. This is a huge infrastructure opportunity. We  should redesign our road hierarchy with these axioms and the possibility of slow vehicles becoming mainstream or at least standard. We should think about developing an interconnected slow vehicle network so that small neighborhood vehicles (think souped up golf carts) cannot not only travel within neighborhoods or on campuses, but between adjacent neighborhoods, without attracting longer distance traffic, where slow and fast vehicles need not mix.

There should be interesting designs for this, which are not today’s standard recipes, since this is as much at the level of network design rather than road design.

3 thoughts on “The Hierarchy of Roads: 7 Axioms on street design

  1. David,
    Really like this post. Can’t resist sharing one of my favorite axioms… all other things being equal, I see no audience for which ceteris paribus is the most appropriate term. Tim

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  2. Dave, very lucid and very clear – some thoughts.

    A former and respected colleague felt (and feels) VERY strongly that trips (be they on highways or by transit) should be separated by trip length as much as possible.

    Hence “local” and “express” trains on some parts of the NYC subway network and “local” and “express” lanes on some urban and suburban freeways (and there may be some exurban/rural freeways that could benefit as well, my favorite being truck-choked I-81 in Virginia).

    The lack of “express” tracks on some rail transit systems is a continued source of annoyance to some patrons (but the cost of those may not be justified).

    It should also help to keep traffic that does not want to be on local streets on freeways instead; and in my fantasy world, all freeways in areas prone to congestion would have some sort of dynamic pricing imposed.

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