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Dr. Gear Head

Don't Drive so Close to Me

November 19, 1998

A shade over 95 feet a second. That's how fast going 65 mph is. And if you're a left-lane banshee who considers 85 a minimum, you're covering nearly 30 feet more than that with every tick of the clock.

So why are you only 40 feet behind the person in front of you? Because you're in a hurry and that person is in the fast lane of the freeway going slower than you want to be going.

What should you do? Well, you could flash your brights, but contrary to popular myth, that's not legal unless you're going to pass on the left on a two-lane road. Now, it is the law in California that slower traffic should travel in the right-hand lane, but that means slower than the speed limit, not slower than when your car runs out of throttle.

If you're really in a hurry and the person ahead of you isn't moving fast enough, your best option is to go around. Passing or overtaking on the right is legal on divided, multilane highways. And it's certainly a better idea than trying to menace somebody out of the way.

Yet you're so focused on driving within spitting distance of the car in front of you that you've left yourself far less room to maneuver. Consider these two things: One, unless you're in something that brakes exceptionally well (read: Porsche or Ferrari, not any SUV), you'll hit the car in front of you in an emergency. Two, it will be your fault.

If that isn't enough to convince you to leave some breathing room, consider why you're in this situation to begin with: You're in a big, fat hurry!

Do yourself and others a favor and leave more room. Relax more. And if those poor folks in front of you can't change lanes, what's the point of driving right on top of them? At least give them a chance to move over. If they do move, reward their courtesy by not making angry gestures.

If they don't? Check all your mirrors every 15 to 30 seconds and look for the opening that allows you to go around--safely, and not at risk of sucking the paint off the side of the other car.

Whatever you do, pick your moves carefully. There's a good chance that if you stay where you are, you'll get there within seconds of when you would have if you had opted to race through the pack. Who hasn't observed someone whose impatience produced a temporary gain, but in a minute or two fell behind those who held their positions and went with the flow?

The law in California doesn't state specific distance or formula. It merely says: "The driver of a motor vehicle shall not follow another vehicle more closely than is reasonable and prudent, having due regard for the speed of such vehicle and the traffic upon, and the condition of, the roadway."

Once upon a time, safe following distance was given as something like 10 to 12 feet for every 10 mph of speed. At 65 mph, that's 65 to 78 feet. The modern formula, which is easier to figure and easier to determine on the road, is two seconds of braking time on dry pavement. At 65, that's 190 feet; at 60, it's 176. That's enough room to stop from 60 mph for all but one vehicle listed in Road & Track magazine's Road Test Summary. And that's a worst-case scenario because you're probably not braking to avoid a stationary object, but it's not unheard of for one to appear. Say, a jackknifed semi.

Complicating all of this is the question of reaction time (see more on this below) and the fact that braking performance isn't linear. Let's say you own the ever-popular Honda Accord. At 60 mph, it stops in 141 feet, so you might figure it can stop from 80 in 188 feet. You'd be wrong by 77 feet.

But, Doc, you argue, I own a really hot sports car and it has really mega brakes!

Yeah, well, the Porsche 911 turbo can stop from 60 mph in 116 feet, but from 80 the distance jumps to 199 feet. That's the shortest distance on R&T's summary for a production car (and only shared with two even more exotic rides). Most cars need an additional 90 to 110 feet more to stop from 80 than they do from 60. Not surprisingly, SUVs and minivans need even more space, sometimes as much as 130 to 140 additional feet.

Getting back to safe following distance: At 80, the two-second distance rule would be 235 feet. But if you're driving the average car or an SUV or a minivan, you're too close to stop safely. Better count three seconds between you and the car ahead.

A lot of factors influence how well brakes perform: road and weather conditions, tires, suspension, anti-lock or regular brakes and, most important, driver ability.

Consider that even if you see the emergency before the driver in front of you reacts, it will take you 0.75 to two seconds to take action. If you don't see the hazard before the driver in front of you, your reaction time basically doubles.

Obviously, there can be situations where you might be closer to the car ahead of you than you'd like, and most likely someone will move in front of you if you leave two seconds of space. But if you position yourself so you have an out, this stuff might not be such a big deal.

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