Engineering New Traffic Patterns, and New Lingo For Them

Technically speaking

3 min read
Traffic is the expression of human purpose.
--Tom Vanderbilt, Traffic

On residential streets, engineers attempt to control car speeds by installing traffic-calming devices . You probably know all about the venerable speed bump , but these days you’re more likely to drive over the lower and wider speed hump or the even wider speed table . Many people refer to these generally as sleeping policemen , a richly evocative, perhaps even poetic locution.

In 1861, when the few steam-powered automobiles around were known as road locomotives (or light locomotives), the British Parliament established the world’s first speed limit of 10 miles per hour (16 kilometers per hour). In 1865 a revised law slashed it to 4 mph in the country and 2 mph in towns and villages. Apparently alarmed by even these modest speeds, Parliament also decreed the world’s first traffic-­calming devices: each vehicle had to be preceded by a person walking 60 yards (about 55 meters) ahead and waving a red flag to warn others of the vehicle’s approach.

The world has seemed like a too-fast place ever since, and today’s traffic engineers spend much of their time getting people to slow down. Along the way they’ve developed a remarkable lingo.

One traffic-calming technology popular of late is the neckdown (also called a bulbout ), which is a curb that extends into the street on both sides of an intersection; narrowing the intersection forces cars to slow down. A similar idea is the chicane , an S-shaped curve that creates curb extensions drivers must negotiate. Speed bumps, humps, and tables fall under the category of vertical deflection , while neckdowns and chicanes are horizontal deflections .

For many people, anything that forces motorists to slow down is a good thing. Hence the popularity in some places of the mobile speed bump , which is a car that travels at the speed limit to force the cars behind to do the same. Tom Vanderbilt, in his terrific book Traffic: Why We Drive the Way We Do (and What It Says About Us) , mentions that the notoriously reckless drivers of New Delhi are often forced to slow down thanks to a natural traffic-calming device: the occasional cow lounging in the middle of the road.

Traffic-calming features aren’t universally loved, however. They make driving a hassle; moreover, some drivers accelerate between humps in an apparent attempt to make up for lost time, a problem for pedestrians. First responders hate the way they are slowed in an emergency. (One novel solution to this last complaint is the speed cushion . Picture a single speed bump split into three smaller bumps; the spaces on either side of the middle bump are just far enough for the wheels of emergency vehicles to pass through, while narrower cars must still negotiate at least one of the bumps.)

To solve these problems, traffic engineers are coming up with novel ideas that create psychological traffic calming. For example, large trees near the side of the road, lowered curb heights, and even a child’s bicycle parked by the roadside create uncertainty in the mind of the driver, and that uncertainly causes the driver to slow down. In other words, it creates a kind of mental speed bump in the mind of the driver.

Some engineers have taken this idea to extremes and created naked streets , which have no signs, road markings, or traffic lights. This sounds like a recipe for car chaos, but by forcing drivers to pay attention to their surroundings, the resulting streets are safer than traditional streets festooned with lights, signs, and other street furniture .

A similar idea is the complete street , a street designed to accommodate cars, public transit, bicycles, and pedestrians. A common feature is the pedestrian scramble , a traffic-light-and-crosswalk system that stops cars in all four directions.

In 2007 more than 41 000 people died in traffic-related accidents in the United States alone, and that’s the lowest number in recent years. We need more sleeping policemen, more naked streets, and, perhaps, armies of volunteers walking 55 meters in front of each car, waving red flags.

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