Designing Streets for Safety

By Dom Nozzi

In September 2007, a citizen in Micanopy, Florida asked me about whether a large “vision triangle” was a good idea at a street intersection. A vision triangle is an imaginary triangle drawn at the corners of an intersection that is to be clear of “visual obstructions” such as buildings or fences or signs.visibilitysmall

I informed this person that in general, the need for a vision triangle arose in the 20th Century, when engineers began to design “forgiving roads” to try to safely accommodate higher speed, reckless, incompetent, inattentive driving of cars and trucks. Such a design paradigm assumes that drivers will always drive at high speeds in a reckless, incompetent, inattentive way.

I believe this concept is increasingly being seen as flawed. It is a self-fulfilling prophecy that inevitably results in the growth of higher speed, reckless, incompetent and inattentive driving.

Drivers drive at the highest speed at which they feel safe. They also devote as much attentiveness as is deemed necessary, and no more. Therefore, if we design roads that “forgive” high speed and inattentive driving by moving trees/vegetation/buildings away from the street with a large vision triangle, a growing number of drivers will consequently drive at higher speeds and less carefully, because they can now do so more safely.

There is now a growing call, which I strongly support, that suggests we should reverse this safety strategy. After all, the track record of this form of conventional safety engineering has been awful over the past several decades. Crashes have not declined, and drivers drive faster and more recklessly.

The new paradigm, which I believe will result in a safer transportation system, is one that obligates motorists to drive carefully. To drive more attentively. More slowly.

In a compact, walkable town center, which needs to be low-speed in design, we need to return to the tradition of designing for the Attentive Street, not the Forgiving Street. Buildings, vegetation (particularly street trees) and fences needs to be moved close to the street. Vision triangles should be quite modest in size.

The dilemma is that at first glance, it would seem that a larger vision triangle promotes safety, since the motorist can see a larger area of potential threats such as other motor vehicles on the crossing street. But counterintuitively, the opposite is true, as I demonstrated above.

Woonerf in Binghamton

 

For safetly, motorists should find streets that require them to drive slower

and more carefully in order to feel safe. By being obligated to be slower and more careful, safety increases.

This is the opposite of what engineers have been trained to believe over the past several decades, so many continue to ferociously oppose such a design shift.

In sum, assuming we are designing a street that would appropriately have a relatively low design speed of 10-20 mph, I would urge the vision triangle to be extremely small. Larger triangles encourage less safe, higher speed, less attentive turns and cruising speeds by motorists.

Would Stop Signs Increase Safety?

In general, stop signs are not a recommended treatment for slowing cars or increasing safety (or making a larger vision triangle safer). Stop signs can create a false sense of security and are often disregarded. Better treatments would include horizontal, physical interventions such as a traffic circle, a roundabout, narrower travel lanes, bulb-outs, on-street parking, speed tables, etc. In other words, as I noted above, designs that obligate the motorist to drive more slowly and attentively. Stop signs do not do that.

To achieve this slower, more attentive and safe street design, it is important to work with an engineer who is sympathetic to the Attentive Street design paradigm. I recommend designers such as Michael Ronkin, Dan Burden, Michael Wallwork, Walter Kulash, or Ian Lockwood.

Low design speeds not only improve street safety. They also improve retail health. They create “drive-to” shopping streets rather than “drive-through” escape routes.

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Visit my urban design website read more about what I have to say on those topics. You can also schedule me to give a speech in your community about transportation and congestion, land use development and sprawl, and improving quality of life.

Visit: www.walkablestreets.wordpress.com

Or email me at: dom[AT]walkablestreets.com

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