Towards a Functional Classification Replacement

The essay "Towards a Functional Classification Replacement" is online in three parts at the Ped Shed blog. Part One covers the background, characteristics and drawbacks of functional classification, and evaluates some of the leading alternatives. Part Two continues by proposing a replacement, a sustainable transportation network classification, covering the block-scale and neighborhood-scale relationships. Part Three concludes by covering the city-scale relationship and the congestion-related impacts of a sustainable network.

From Part One

Every field has its foundational working concepts and the field of traffic engineering is no exception. It has a concept called functional classification, which is the core, guiding idea underlying the roadway system of the United States and many other nations. Functional classification is the conceptual foundation of the auto-dependent built environments where most Americans live.

The primary vision of functional classification is moving more and more cars at faster and faster speeds. This has certain benefits, but also a wide range of disastrous consequences for the built environment and the people who live in it. Hundreds, possibly thousands of reform-minded transportation planners and engineers have determined that the roadway functional classification system should be replaced.

It should be replaced by guiding concepts that support a more efficient, safer, less-polluting transportation system - concepts that support a wider range of choices for neighborhood living and daily travel. What factors should be considered when formulating a sustainable transportation system? What proposals have already been made?

From Part Two

A sustainable network classification ideally will do several things.

* Actively encourage sustainability (as defined previously in the sustainable transport section); do not support unsustainable network patterns and operations.

* Be concise, easy to remember and easy to explain.

* Address a range of scales, a range that is at least as wide as that covered by functional classification.

* Incorporate advanced knowledge about network function and best practices in network planning.

To reach these goals, a sustainable network classification is proposed. The classification has three primary relationships, each applying to a different scale. The three scales are block scale, neighborhood scale, and city scale. This allows each relationship to focus on the factors most relevant to its scale, without unnecessarily confusing factors from different scales or combining them inappropriately.

From Part Three


The sustainable network classification proposed in this essay reflects a vision of ideal patterns and principles. It does not attempt to accommodate or compromise with existing practices that do not contribute to more humane and sustainable built environments. This is undeniably liberating, but also may sideline the effort as trivial and excessively idealistic.

The sustainable network classification consists of the following elements:

1) Connectivity and place accessibility are prerequisite conditions for the block, neighborhood and city-scale relationships.

2) Block scale relationship: person-capacity to place context. The multimodal capacity of thoroughfares is related to walkable design elements that are coordinated on a rural-to-urban spectrum of contexts.

3) Neighborhood scale relationship: network accessibility to land use movement sensitivity. The accessibility of thoroughfare segments is related to the requirements of various land uses for adjacent multimodal traffic, in order to create viable neighborhood structure.

4) City scale concept: network interrelationships organized by settlement scale. A framework of settlement scale organizes a nexus of relationships between networks and between network scales.

5) All of the above conditions and relationships must be considered concurrently and coordinated the maximum degree possible.

This essay is only one step towards a fully elaborated and tested sustainable network classification. It can undoubtedly be improved upon. Networks overlap and interact in complex ways and it is fair to say that no one yet fully understands all the interactions and feedback loops between transportation networks and settlements across all scales. The author's hope is that this essay will begin new dialogs and give added impetus to ongoing discussions. Questions, corrections and discussion are requested and welcome.

-Laurence Aurbach


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