Wednesday, December 18, 2013

The Forgiving Design Hierarchy

While this post started as a fork from Compromises on North Broad, it is really a topic unto itself, and a core post of my thinking on how design affects safety. I am calling it (in draft) the forgiving design hierarchy concept of street design workflow. I hope that, as you read, the reasons will become clear.


The key is attaining a tradeoff--a balance of priorities. To do this we need some sort of hierarchical modal evaluation criteria. This could be easily abused and turned into a checklist by the same kinds of numbnuts who eviscerated any pretense of urbanism in Syracuse, but having such a checklist exist, and be public knowledge, would be an invaluable tool in project evaluation and project criticism. For a street--a value-creation platform--modal consideration would progress
  • Pedestrians
  • Cyclists
  • Dedicated Mass Transit (if applicable)
  • Motorized Transport
in that order. This is tied to an underlying principle that design consideration scales with modal vulnerability--that is, the more vulnerable a given user is, the greater the design consideration for them should be. This, of course, does not preclude high-quality facilities for the dominant (and in road situations, usually the only) mode of travel; rather, it merely states that first attention should be given to a facility's most vulnerable users. This does not really affect the layout of true roads at all; indeed, it subtly enforces the road/street division in transport hierarchy.

The Role of Forgiving Design
What we call "forgiving design" is the set of auto-oriented transportation engineering design guidelines that first emerged with the initial construction of the Interstates. It is an extension of rural design standards, applied with the idea that such standards would improve speed and flow in urban areas. That it is should surprise no one: as I have just pointed out, even in my revised system, most rural byways would stand unaltered. Unfortunately, urban areas, busier and more congested (both good and bad), are hence more incident-prone. The problem is, of course, that current forgiving design principles have proven to be a failure at increasing safety in urban areas--and, even worse, have proven to be a disaster from a safety standpoint in suburban areas (cf. Dumbaugh and Li).

Subjective Safety

Key to the performance of the Dutch cycling network is a concept called subjective safety, the realization that, when it comes to driving usership, the perception of safety is just as important as what is, statistically, "safe". So the optimal approach to generating the prioritization hierarchy above would seem to be to implement subjective safety principles across the more vulnerable modes, fitting forgiving design for automobiles in where it can fit (precisely the opposite of our current approach, which fits in infrastructure where it can fit after the cars are designed in). But--at least in London--subjective safety and forgiving design appear locked in battle, with TfL's flow needs trumping subjective safety needs, despite the lobbying of numerous cycling-advocacy organizations.

Two Sides of the Same Coin

But the principles of subjective safety are the principles of forgiving design--just applied to a different mode. The whole point of subjective safety, the perception of safety, is achieved by creating an environment forgiving of error; the whole point of forgiving design is an environment forgiving of driver error. This intermodal mesh drives the British conflict: on London's limited road space, who gets the space allocation they need for their approach? Who "wins"? Are they the drivers, accommodated by forgiving design, or the cyclists, accommodated by subjective safety? But it doesn't have to be this way: understanding that they are one and the same allows us to integrate the two elements--forgiving design and prioritization hierarchy--into a single model.


Subjective safety and forgiving design, now revealed to be one and the same thing, merely tell us how to design safe* infrastructure for a particular mode. But our world is multimodal. To answer the question of which mode gets priority, the hierarchy, increasing attention for increasing vulnerability, yields the answer. Hence the framework which I am proposing, and wish to investigate further, is one where we
  1. Consider a given route. Consider factors such as existing usership(s), current population density, planned population density, distance/mode optimization, origin/destination proximity, etc., to model usership. For roads, none of this matters. But roads also cannot have curb cuts.
  2. Design for each user in turn of vulnerability.
  3. For each mode, apply forgiving design concepts. For places with low pedestrian volume, but increasing bike volume, for example, a multi-use trail will work.
  4. Evaluate whether or not the tax base can support the design. If not, reduce until within this "solvency envelope".
which is, of course, largely alien to current transportation engineering workflows.

Further Questions

This is just a framework. Many unanswered questions within this framework remain, such as what minimum adequate forgiving design for each mode would be, what optimal infrastructure in several contexts would look like, and even the whole question of designing for enduring financial solvency. In fact, this framework is the core of the research I wish to pursue, largely because I find myself convinced this is the right approach and would like to see it applied and further refined--but as it stands, it is hardly ready-to-go as a framework, and is rather much more conceptual.
* With certain exceptions, e.g. suburban arterials, where forgiving design appears powerless against an even more foundational foe, the curb cut.

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