On Design Fundamentalism and Speed: Debate on Vision Zero Principles

Over at our Strong Towns group there's a link to an interesting note at Strong Towns HQ.

Strong Towns on weakness of signs alone

Just a few days ago, legal scholar and safety advocate Greg Shill suggested, by contrast, that design fundamentalism was an error. A link to that piece was also posted to our Strong Towns group.

Greg Shill against "design fundamentalism" (bluesky)

And in response to Shill, safety and transportation journalist David Zipper suggests Shill might be strawmanning the argument a bit, saying "I think he misrepresents and undersells Vision Zero."

David Zipper: Shell misses on Vision Zero (bluesky)

And finally, here in Oregon there's a body of advocacy that appeals to the City of Portland's data on reductions in "top end" speeding after they simply changed a bunch of signs.

But the Portland data might not be as strong as it seems.

City of Portland overstating "sharp decreases"

While they suggested "sharp decreases" occurred it might be more accurate to say modest and small decreases in speeding occurred. The percentage change (the "change in odds") looks more dramatic that the raw counts would look, and a change from 1.1% to 0.6% may not be much more than a rounding error or any margin of error, though in the total pattern of reductions it likely has some significance — though again, modest rather than sharp. To say -49.6% about the difference from 1.1% to 0.6% might be mathematically true, but it is not the best and most useful way to describe the change.

All this is a fruitful conversation. It also points to multiple elements of causation, and the necessity of systems thinking for solutions. But the self-enforcing nature of better road design is more durable, and operates when there are no police around, than signs and police stops.

By itself road redesign is costly, but by itself swapping out signs is nowhere near enough. Increasing enforcement is necessary, but the scrupulous neutrality of camera enforcement is superior to the biases that seem inherent to pretextual stops by police. There's no magic single solution, and safe systems will have to be layered. It's important to insist on the layering.

The heirarchy of controls might be a better framework than...

Thinking about safety mitigations - via former twitter
see also "heirarchy of controls" at wikipedia
and at the CDC

...our recent safe system pie chart from the Metropolitan Transportation Safety Action Plan. Prevention in safer roads should be given greater weight than post-crash care, for example, but the pie slices suggest a rough equality of weight.

Our final pie from last September's MTSAP

It is good to have these discussions, and because it is true that road redesign is often costly, "second-best" solutions will remain very useful. But we should not be misled that slapping signs and enforcement on a big, wide stroad will make it slow and safe.