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How Human Perception Shapes Highway Design

By Wesley Fenlon

We drive fast on the highway, but we're rarely cognizant of just how quickly our cars are moving, as the design language of roadsigns and lane lines skew our perception of reality.

Imagine a highway stretching out endlessly before you, the white lines dividing lanes of traffic evenly spaced into infinity, and you'll probably be recalling a vivid memory. A cross-country road trip with friends. A childhood vacation. The end of of Terminator 2. In those memories, how long are the dashes that skip by our windows every few seconds? A foot? Two feet? Four?

According to psychology researcher Dennis Schafer, the most common answer is two feet. It is, then, also the most common wrong answer, as the highway guidelines in the United States recommend that those lines be ten feet long, with gaps of 30 feet creating the empty space between them.

Photo Credit: Flickr user Nicholas_t via Creative Commons.

Roman Mars' podcast 99% Invisible dedicated a recent episode to human perception of the open road, and how skewed that perception is when it comes to our experiences of size and speed. We look so far down the road that we rarely grasp the true size of objects; only when we encounter objects up close, like another car whizzing past us in the opposite direction, do we realize how fast we're moving.

The episode includes guest Tom Vanderbilt, who wrote a book titled Traffic: Why We Drive the Way We Do (and What it Says About Us). The Kindle version costs $14. Vanderbilt talks about the ways our roads shape us and the way we've shaped the roads. Those dividing lines aren't always 10 feet long--some are even longer--but in congested urban areas, where we drive more slowly, the lines are naturally shorter.

My favorite part of the short podcast touches on what happens when the design language of the highway carries over to suburbia, where shopping malls and fast food places bracket the roadways. Here our perception and instinct propel us to drive fast, while signs beckon us to stop and shop. Turns out, they're some of the most dangerous roads to drive in America.