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Consider the aerial photo below.  We spent some quality time in this neighborhood (Chelsea & Meatpacking) last weekend.  I’ve overlain a prominent feature in my life these days, the intersection of Nutley St. and I-66, near Fairfax, Virginia.  If I lived further out, I would know this as exit 62 on I-66.  Fortunately, I don’t live that life.


Lanes and Space

Traffic requires much more space per head than walking: about 1,100 SF compared with 20 feet.  This is partially because of the need for parking, but mostly because of stopping distance.  The average speed of a car is 31 miles per hour, while the average speed of a pedestrian is 3.  Because the road is a flat and free surface delimited by paint suggesting where to go, the lane width allotted to each car needs to be wider the faster traffic is. The fasgter a car is going , the quicker it can get into trouble with a moment’s mis-steering.  The lanes on I-66 are 12 feet wide, twice the width of the cars traveling on it.  This allows cars traveling 88 feet a second to steer a bit sloppily without colliding with other cars or much heavier trucks on the highway.

While the reason for 12-foot lanes is speed, 12 feet is the “suggested” width for traffic lanes all the way down to 25-mile-per-hour roads.  The lane width in places like Chelsea and many older neighborhoods is 10 feet or less.  One of Atlanta’s most desirable neighborhoods (Virginia Highlands) has roads so narrow every passing is a negotiation with eye contact, stopping and pulling over into the parking lane.  For new roads, there’s no reason highway designers shouldn’t specify 12 foot lanes. Its the standard; the default.  The most expensive thing about building these roads is mobilization and labor.  Once you’ve got the crew out there, you may as well build to the full width of your equipment to proof against collisions.  Even if the volume and speed of traffic is a residential trickle.

As for sidewalks, walkers require 2-3 feet of lane, depending on if they are in wheelchairs or not.  Walkers weigh about a fiftieth of what a car weighs, so the sidewalk doesn’t have to be built to the same standard as the roadway.  And colliding with somebody, while rude, is nowhere near as catastrophic.  Andres Duany quipped that “it is much different to run into somebody on the sidewalk than it is in a car.”

The capacity of a minimal 5 foot-wide (2 “lane”) sidewalk (4,000 people per hour) is greater than the capacity of a 12 foot traffic lane (3,200 people per hour).  Of course, no one’s having much fun at those densities, but it is interesting to know what can be done with our real estate. It is also interesting to note that traffic engineers get itchy to add a lane when the throughout on existing lanes exceeds 1,300 people per hour. Less than half the capacity of the lane. I don’t have practices on when to widen sidewalks, as they are used so rarely in America they are almost never widened.

Space and Time

Consider walking in a trafficked place.  Most people in traffic look at sidewalks as underused, even wasteful things.  Surely they could widen the curbs by one more lane in the congested areas and difficult intersections, since we almost never see anybody on those sidewalks.

Requiring parking and setbacks for traffic makes everything spaced out.  Consider that 40 cars (160 people) require an acre to be in traffic, only 34 cars can drive and park in a place per acre, a road lane is 12 feet wide, and the stopping distance is around 100 feet for a car.  All that requires space.  The rate of walking is 4.5 feet per second.  It takes 2 seconds to cross one parking space, 3 seconds to cross a traffic lane, 4 seconds to walk the length of a parking space, 12 seconds to cross a parking lane, and most of a minute to cross that square acre.

In automotive places, the first thing a business needs to indicate to their audience is that there is parking to store their cars and trucks in.  Without parking you cannot go there, at least in traffic. A parking lot up front is the most obvious and welcome sign of “Home” for people in traffic. For walkers in these “places”, the thing between them and the continuous, unfenced, and relatively car free sidewalk is a few rows of parking.

Of course, every new car in traffic makes traffic worse. So making your city in the image of the car, so that only fools, destitutes, and suicides would consider thinking about walking or biking there, only means your traffic will get worse. Zoning cities and regions so that all the good jobs are far away, and even the fair jobs are too far away from the housing to walk. When everything is designed for the car, don’t be surprised when everyone gets in a car. Which of course requires that we build yet more for the car.


If you want walkers to walk on your sidewalks, to be seen in public and stay out of traffic with their own cars, make places worth walking to.