Standard car lanes are 20% too wide

Your local paving crews could making streets safer every time they resurface.

“Even if we had the budget for traffic calming, you can’t reduce the lane widths on arterials. 12 feet is the requirement.” —Status Quo Engineer

I’ve had the good fortune of working for some consulting firms over the years who did quite a bit of safety research. They were concerned with reducing crashes, even if the findings were counterintuitive. Thanks to them, I was exposed to lane widths and road diets fairly early in my career.

Open minds are the exception to the rule in this industry. Whether by accident or by design, you’re being lied to about lane widths.

Skinny lanes, safer streets

Reducing the width of car lanes might be the easiest, fastest, cheapest way to improve traffic safety. But it's treated like some kind of extreme re-do that requires everyone's useless input. I’m talking about the ordinary suburban and urban streets, not interstates. 

In population clusters, wider traffic lanes are more dangerous than narrower lanes. Cities with travel lane widths from 9-10.5 feet (e.g. Amsterdam, Copenhagen, and Tokyo) have the lowest crash fatality rates per 100,000 residents.

Every time the crew goes out to resurface or repaint, they could be lane dieting.

Wider lanes are often perceived as safer because they allow space to swerve back into line when you drift. When you’re on the interstate, steering with your knees while balancing your Panera or Chick-Fil-A, there’s comfort in knowing you have wiggle room. Careful highway drivers also appreciate wide lanes because at high speeds, just a flick of the wheel can cause a sideswipe crash. 

Vehicles traveling in opposite directions on undivided, two-way roads are separated by larger distances if lanes are wider, reducing the risk of head-on collisions. We’ve all been on a country road that felt scary when someone flew by in the opposite direction. 

But in an urban or suburban setting, you don’t want drivers moving at 55 mph. You want speeding to feel uncomfortable. Contrary to conventional engineering practice, lane widths of 10 feet are not only appropriate, but beneficial.

This is the point to drive home and the one constantly ignored by transportation engineers: some streets need slower vehicles.

NACTO on Lane Width

National Association of City Transportation Officials (NACTO) provides comprehensive guidance on various aspects of urban street design, including vehicle widths and buffers. 

NACTO makes an important step by recommending 10-foot lanes as the default in cities:

Lane widths of 10 feet are appropriate in urban areas and have a positive impact on a street’s safety without impacting traffic operations. For designated truck or transit routes, one travel lane of 11 feet may be used in each direction. In select cases, narrower travel lanes (9–9.5 feet) can be effective as through lanes in conjunction with a turn lane.

They found that a narrower lane width, which can go down to 10 feet or even narrower in special cases, is associated with safety benefits for all users. The rationale is that narrower lanes generally result in slower travel speeds, and that no matter how you get around, slower vehicle traffic keeps you safer. From a geometric perspective, narrower lanes means a shorter crossing distance for pedestrians. Less exposure, less risk.

FHWA on Lane Width 

The Federal Highway Administration (FHWA) research indicates that road geometry has a significant impact on traffic safety. Their words with my emphasis:

The primary purpose of traffic calming is to support the livability and vitality of residential and commercial areas through improvements in non-motorist safety, mobility, and comfort. These objectives are typically achieved by reducing vehicle speeds or volumes on a single street or a street network. Traffic calming measures consist of horizontal, vertical, lane narrowing, roadside, and other features that use self-enforcing physical or psycho-perception means to produce desired effects.

Lane narrowing is an important feature of traffic calming, and traffic calming is so high on FHWA’s priorities that they have a slew of webinars and guidelines about it. It’s one of their proven safety countermeasures. 

AASHTO on Lane Width

The most unpopular opinion held by the American Association of State Highway Officials might be that engineers are expected to exercise good judgment. Of course, that shouldn’t be rare for licensed professionals since they swore to hold public safety paramount. 

AASHTO’s “green book”, a guide often used as an excuse for highway-feeling design, includes road design tables with recommended lane width ranges. 10 feet is allowed for the type of arterials we all know to be dangerous for walking, bicycling, and driving. There are multiple tables for multiple land use contexts, and that’s where engineering judgment is supposed to come in. 

Transportation Research Board on Lane Width

TRB is an organization that publishes projects and studies from around the world. Two notable TRB studies found no consistent relationships between lane width and safety on urban and suburban arterials. This finding is noteworthy because it contradicts the common engineering claim that 12-foot travel lanes are safer than 10-foot lanes.  

Here’s a summary of Safety Effects of Cross-Section Design on Urban and Suburban Roads. And here’s a very technical paper if you like a challenging read: Relationship of Lane Width to Safety for Urban and Suburban Arterials. But here’s the bottom line from TRB research that every engineer should understand:

It is recommended that narrower lane widths be used cautiously in these situations unless local experience indicates otherwise

• lane widths of 10 ft or less on four-lane undivided arterials.

• lane widths of 9 ft or less on four-lane divided arterials.

• lane width of 10 ft or less on approaches to four-leg STOP controlled arterial intersections. 

Now what?

The suggestions above will sound radical for a status quo transportation professional. But these aren’t childish daydreams, they’re the findings of rigorous research. Standard car lanes are 20% wider than they should be.

Skinny the lanes to save lives. Go forth and diet.