RAY MAGLIOZZI

Dear Car Talk: As I’m sure you’re aware, for many years, there was a rule for following distance — namely, you should leave one car length between you and the car in front of you for every 10 mph of speed. Now, however, the so-called “2-second rule” is recommended, and I have read online that the National Safety Council now recommends a margin of 3 seconds.

The change in the rules occurred sometime during the 1970s. Do you have any thoughts as to why the rule for following distance was changed? — Tom

I thought the 3-second rule had to do with dropping a piece of buttered toast on the floor. I can take an educated guess as to why the “one-car length for every 10 mph” rule was abandoned.

A car length is about 15 feet. So if you’re traveling at 65 mph, and leave one car length for every 10 mph, you’d leave a little less than 100 feet between you and the car in front of you on the highway.

How long does it take the average car to stop when it’s going 65 mph? About 300 to 350 feet. That includes the 140 feet you’d travel at 65 mph before the average human could even react and step on the brake. So, the reason the “one-car length” rule is never cited anymore — except maybe by the collision repair shop owners’ association — is because it’s wholly inadequate.

What about the 3-second rule? That’s where you pick a road marker, and make sure you reach that marker three seconds after the car in front of you passes it. In 3 seconds, a car traveling at 65 mph would go 285 feet. That’s better. And if the car in front of you slows down (as opposed to stopping short for an obstacle or crashing into the car in front of it), you’ll have a much better chance of being able to brake and avoid getting intimate with the logo on the guy’s trunk lid.

Of course, it’s getting harder to maintain 3 seconds of space at 65 mph. Many of our highways are so crowded now that if you leave a proper amount of space, someone will instantly cut in front of you and cut your safety margin in half. You might eventually have to go backwards to maintain it.

I guess the good news is that collision avoidance systems like highway-speed automatic emergency braking should help by having quicker reaction times than we humans do. Especially since your car’s sensors aren’t trying to find the fan blower adjustment submenu on your center display.

But even they can’t overcome the laws of physics. So, if it takes your car 300 feet to stop and you’re 200 feet from a stopped car, prepare for liftoff. While 3 seconds may not always be adequate, it’s a lot better than one-car-length-per-10 mph, Tom.

Got a question about cars? Write to Ray in care of King Features, 300 W. 57th St., 41st Floor, New York, NY 10019, or email by visiting the Car Talk website at www.cartalk.com.