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How wide should car lanes be?

A slight change in where lines are painted is a matter of life or death.

Of all street design elements, no other has evoked as much bafflement, incredulity, and conjecture as the safer range of travel lane width. Traditional traffic engineers argue wider lanes are safer. Supporters of the livable street concept passionately promote the safety benefits of a relatively narrower lane width. —Dewan Karim

The video above is from a series called Walk Lobby TV that I made way back in 2015. This one hits on a very important design detail that’s overlooked all the time.

Lane width is treated as a constant, not a variable, in street design equations.

There’s only so much space between curbs, and (12 ft) x (N lanes) is just how it goes. When I started using traffic analysis models in the 90s, 12 feet was the default. The only time we were allowed to tweak the setting was if it helped improve level of service (measured in the number of seconds you wait for green).

What I quickly learned from the traffic modeling software was that adjusting the lane width from 12 down to 11 or 10 would add some seconds of delay for cars at an intersection. That was considered bad. But the reason the answer was “bad” was because the model was calculating that people drive slower when car lanes are narrower.

Skinny streets didn’t bring gridlock, they calmed traffic. And by calm, I mean lower speeds and fewer crashes.

In other words, standard engineering practice does not use engineering judgment. Imagine having two options for lane width defaults, Dangerous or Safe, and making the wrong choice every time.

Quick refresher on the outcomes of making infrastructure decisions based on shaving seconds for car traffic:

Can 12-18 inches really make that much of a difference?

Yes! It turns out drivers naturally lift their foot off the gas when lane stripes get narrower. It’s a sort of driving claustrophobia that heightens a driver’s awareness. The feeling of discomfort makes people better drivers.

States and counties believe that wider lanes are safer. And in this belief, they are dead wrong. —Jeff Speck

Old cities like Washington, DC, Boston, and San Francisco have 8- and 9-ft lanes in many neighborhoods and they work just fine. But over time, local governments have been widening streets based on traffic engineering models. After lanes are widened, agency heads (and their consultants) will claim safety as the reason. They’re coming up with an excuse that sounds good to justify poor judgment.

So how narrow can car lanes be?

Studies are piling up saying 10 is a great number. It might not sound like much, but 10 or 10.5 feet is much safer than 12 feet. In lieu of Thanos’ gauntlet, engineers wield the AASHTO Green Book to defend wide lanes and snap out of existence bike lanes.

  • “There’s just not enough space for bike lanes because the vehicular lanes need to be 12-feet wide.”

  • “I’m sorry the extra lane width makes it so dangerous for pedestrians to cross the street, but the vehicular lanes need to be 12-feet wide.”

  • “Oh, did we mention we need to widen the turn lane to be 15-feet wide?”

Good news for safety avengers: AASHTO says narrow lanes are fine. It’s legal for your city or county to make streets safer by narrowing the car lanes.

Increasing road width to reduce congestion is the same as loosening your belt to fight obesity. —Lewis Mumford

How can all this be true?! I need more evidence!

Follow the links below for more information about the connection between safety and the width of car lanes. It’s kind of a big deal!

  1. Why 12-ft traffic lanes are disastrous

  2. The truth about lane widths

  3. Narrower lanes, safer streets

  4. Context sensitive state design manual

  5. Narrow traffic lanes make cities safer

Urbanism Speakeasy with Andy Boenau
Urbanism Speakeasy with Andy Boenau
Andy Boenau