Q: What’s the appropriate following distance when you’re behind another car? Four seconds? Two seconds? Or a specific number of car-lengths? When does it become tailgating?

A: It depends. That’s not a helpful answer, is it? Washington law states that a driver “shall not follow another vehicle more closely than is reasonable and prudent,” which is a fancy way of saying, “It depends.” What then, does it depend on? The law goes on to say that drivers should have “due regard for the speed of such vehicles and the traffic upon and the condition of the highway.” Now we’re getting somewhere. We should follow at a safe distance, considering vehicle speeds and the conditions of the roadway. It becomes tailgating when the distance isn’t reasonable, but that still doesn’t get to what you’re asking.

Our brains love simple rules. Each time we can reduce a decision to a quantifiable absolute we lessen our cognitive load, so we try to turn, “it depends on multiple constantly changing factors” into “four seconds of following distance.” By the way, to counter what I just said, four seconds is actually a solid starting point.

Using physics, math, and brain science, we can come up with a pretty good estimate of proper following distance for various driving situations. I began that last sentence saying physics and math, so anyone that’s still reading, thanks for staying with me. Don’t worry; this won’t get too technical. Let’s add up all the things that go into calculating a safe following distance.

It takes our brain about a second-and-a-half to process the information we observe and then decide what to do about it. That’s not a hard number. If you’re at the top of your game, it could be as low as three-quarters of a second. If you’re sleep-deprived and stressed out about a work deadline it could be more. Then add in another second or more for the physical action of applying the brake. Now you’re at about three seconds, the minimum recommended following distance. Sure, you might be able to do all that faster under ideal circumstances, but how often do we get ideal driving circumstances?

Before we go further, I should explain how to time your following distance. To measure, pick a marker on the roadway. It could be painted stripe, a pavement crack, or a shadow of a tree. When the car in front of you crosses the marker you picked, start counting “one-one thousand, two-one thousand, three-one thousand.” (You can also use Mississippis or alligators to count your seconds.) However many seconds it takes you to reach that same marker is your following distance. We don’t use car lengths anymore, because who can accurately calculate seven car lengths at 70 mph?

Three seconds works if you’re driving a well-maintained car in good weather. If you’re driving a full-size SUV add a second; heaver vehicles generally take longer to stop. Add another second for speeds above 55 mph. Your brain can only process a limited amount of data, so at higher speeds you’re missing out on information. Add another second or two if it’s raining, a couple more if it’s snowing, and at least two more on top of that if it’s icy. Add even more if you don’t properly maintain your tires and brakes. As you can see, the right distance depends on a lot of factors but, if you want a simple rule, start at three seconds and add a second or two for each additional risk factor. For everyone that doesn’t like that answer, please include math and physics in your counterargument.

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