Design Speed is a Value Statement

traveling at speed of light

Choosing a design speed is, by its nature, an application of core values. When we pick a speed, we are selecting among different, competing priorities. Is it more important that peak traffic move quickly or is it more important to maximize the development potential of the street? Do we compromise the safety of people crossing on foot to obtain a higher automobile speed, or do we reduce speeds in order to improve safety for people outside of a vehicle?

These are policy decisions. Shouldn’t public officials be given the broad range of options and be allowed to weigh them against each other?

Of course they should! So, why aren’t they?

Many of my engineering colleagues will reply that they, the engineers who design streets, don’t control the speed at which people drive and that speeding is an enforcement issue. Such an assertion should be professional malpractice. It selectively denies both what engineers know and how they act on that knowledge.

For example, professional engineers understand how to design for high speeds. When building a high-speed roadway, the engineer will design wider lanes, more sweeping curves, wider recovery areas and broader clear zones than they will on lower-speed roadways. There is a clear design objective (high speed) and a professional understanding of how to achieve it safely.

There is rarely any acknowledgement of the opposite, however: that slow traffic speeds can be obtained by narrowing lanes, creating tighter curves, and reducing or eliminating clear zones. High speeds are a design issue, but low speeds are an enforcement issue. That’s incoherent.

The other pushback often given by professional engineers for why they, and not public officials, should set the design speed is that non-professionals are not qualified to do so. In 2016, I wrote “Engineers Should Not Design Streets,” an article for which many of my fellow professionals accused me of being gratuitously provocative. I was not.

The design of streets begins with the establishment of priorities. It begins with an application of core values. Engineers generally lack the background, training, and understanding to make such a complex decision. Indeed, I think engineers have become uniquely unqualified to do so.

For local streets, setting the design speed is something that should be done only by policymakers (elected officials) and only after a broad and deep dialogue with the community about values and priorities. This is not a decision to be made through the myopic prism of one professional silo. It is too important for that.

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