Imagine for a moment that you are driving on the highway. Perhaps you see a skyline in the distance, but mostly it’s cars, asphalt, and lots of white stripes separating the lanes of traffic. You probably see them every day, but have you ever considered the length of each stripe? Go ahead, make a guess.
If you’re not a transportation wonk, your guess was probably two, maybe three feet… which is not even close. Let me stop here and offer a plug for the 99% Invisible Podcast, which is where I first heard this stumper. Episode 68: Built for Speed should be required listening for city planners and traffic engineers. For everyone else, click thru is highly recommended.
Ok, now for the answer. Ten foot stripes with 30 foot gaps are currently recommended by the Federal Highway Administration’s Manual on Uniform Traffic Control Devices (MUTCD). Some older highways may have longer stripes. Most city streets will have shorter ones. But they are all way longer than two feet. Why do most people get this so wrong? It all boils down to a problem of perspective. When viewed from the driver’s seat, these stripes are subject to foreshortening. In other words, dimensions along the line of sight appear relatively short when compared with dimensions across the line of sight.
This concept comes into play frequently in our work. For example, in a recent motor vehicle case, a witness seated in a pickup truck, observed a truck & trailer collide with a pedestrian directly in front of him. This might seem like the ideal vantage point to observe what happened, but in fact the foreshortened view made it impossible to accurately judge distance to the entry gate where the collision took place. In this case “it happened right in front of me” did not equal strong eyewitness testimony.