Reader Anna C. Martin of Falls Church asked why drivers sometimes stop two or three car lengths from the vehicle in front of them at stoplights. "I find this disconcerting and dangerous," she wrote. "What is the philosophy of this?"

I didn't know and asked the audience. There's no shortage of helpful answers:

Dear Dr. Gridlock:

I have also noticed this tendency for drivers at red lights to leave extraordinarily large gaps between themselves and the car in front of them.

This tendency seems to have started several years ago when carjackings were a more common occurrence. I believe that one of the recommendations made to avoid being carjacked was for drivers leave sufficient room in front of their vehicle so that they had space to maneuver and escape.

While not having seen anyone "jam on his brakes quite suddenly," what I have observed is people starting to brake from quite a distance away from a stoplight and then slowly roll up to within no less than a full car length from the car in front of them.

I find the extra space used in this way most noticeable in situations when trying to make a left turn at an intersection with a raised median. The resulting long line of cars and open space make it impossible to get into the left-turn lane until the through traffic gets a green light and begins moving (by which time, of course, the left-turn signal has turned red).

If one can get within a few car lengths of the left-turn lane, drivers in front will sometimes see the left-turn signal and close the gaps, allowing entry to the left-turn lane.

Brin Tolliffe

Woodbridge

Dear Dr. Gridlock:

In response to the woman who asked why drivers leave two-to-three car lengths between cars at stoplights, I have to admit that I am one.

I don't do this at every light, just at busy intersections where I stand a better chance of being rear-ended by a drunk or careless driver slamming into me, or the car behind me.

This gives me a buffer zone for two reasons. First, I won't be as likely to be slammed into the car in front of me, and second, the car's most expensive end will sustain little or no damage.

I have always done this when driving my antique car, but in recent years, as I have witnessed many more rear-end accidents and endured higher car insurance because of the aggressiveness of drivers in this area, I just subconsciously started this practice with my newer car.

Linda M. Cajka

Lake Ridge

Dear Dr. Gridlock:

I really don't know why they do it. I used to think maybe they couldn't see far enough over the steering wheel, but that can't be it because many of them obviously are tall enough. Could be they have bad eyes and they think they're closer than they really are? Or maybe one time they rear-ended someone and don't want to take that chance again.

Daryl Bickell

Woodbridge

Dear Dr. Gridlock:

In drivers ed at Mount Vernon High School in Fairfax County (many years ago), I was taught by Mr. Foley that one should leave one car length between cars stopped at a light for several safety reasons:

1) If the car in front breaks down, the car behind can move around without having to back up.

2) If the second car is rear-ended, it might not hit the car in front -- saving damage to the middle and front cars and passengers,

3) The second car is less likely to be completely boxed in by potential "evil-doers." Always be aware of an "emergency exit" when driving.

Mr. Foley's suggestion to achieve one car length at a stop was to be able to see the tires of the car in front of you, on the pavement.

Also, back in the olden days, I drove a manual Volkswagen that would roll back several feet before going forward after stopping, so it was much better if the car behind me left themselves some room!

Heather Herndon

Alexandria

Dear Dr. Gridlock:

I always try to leave one car length in front of me at all red lights, for several reasons:

1) It provides some space to maneuver if the car ahead of you becomes disabled or, more important, if someone approaches your car on foot with ill intent.

2) If someone hits you from behind, you stand a better chance of avoiding a collision with the car ahead if you have room in front.

3) We breathe enough air pollution without snuggling up to someone else's exhaust.

Barbara Righter

Alexandria

Dear Dr. Gridlock:

I myself do this at a few select intersections where there is a single lane and no left-turn arrows. I have seen too many cars begin through the intersection and attempt to turn through oncoming traffic (with no turn signal), blocking the cars behind him/her.

I have found that by placing a couple of feet between my car and the car in front of me, I can safely maneuver around the vehicle blocking the path of traffic.

If enough people used their turn signals, I wouldn't have to second-guess the lack of one.

John Masten

Arlington

Dear Dr. Gridlock:

If I have left space between our vehicles, I can then get around the "turning left without signaling vehicle" and cross the intersection, and perhaps even another few cars can do likewise.

Delores Boyer

Arlington

Thanks for your opinions. They have caused me to reassess my own driving. From now on, I'll try to leave a half-car length in front of me at stoplights.

I do worry about folks having access to the left-turn lanes, so I think a half-car length should be all the extra space I should use. If others still can't get in that left-turn lane, I think we should all move up as much as possible.

Transportation researcher Diane Mattingly contributed to this column.

Dr. Gridlock appears Sunday in the Metro section and Thursday in Loudoun Extra. You can write to Dr. Gridlock, 1150 15th St. NW, Washington, D.C. 20071. He prefers to receive e-mail, at drgridlock@washpost.com, or faxes, at 703-352-3908. Please include your full name, town, county and day and evening phone numbers. Dr. Gridlock cannot take phone calls.