Dear Dr. Gridlock:

"Mysterious vibrations on the Wilson Bridge" are causing electrical problems and thus two-hour delays in lowering the bridge span (The Washington Post, July 17). Frankly, I doubt the vibrations are actually a mystery to the traffic engineers. They are understandably reluctant to admit the origin of the vibrations.

Every multi-ton 18-wheeler traveling Interstate 95 gives the bridge a heavy pounding at every expansion joint. No wonder the electrical conduit is falling apart. Even in a passenger car, a ride across the bridge is far from smooth.

Ironically, the contractor responsible was awarded a very substantial incentive fee for doing such a good job. There clearly needs to be a procedure for citizen oversight to unravel some of the "mysteries" that aggravate commuters. DAVID W. PENDLETON Mount Vernon

The problem is not with the contractor or the expansion joints along the road surface, according to the city traffic chief, George Schoene. The problem lies under the bascules, the steel grates that are raised and lowered to permit boat traffic through.

The electrical system, which was overhauled only last year, was found to have partly disintegrated and come apart within steel conduits under the bascules. Why this happened is still a mystery, according to Schoene. "The conduits were located in exactly the same places they were for 25 years," Schoene said. "Maybe it's the increase in traffic. In 1984 we had 125,000 vehicles a day cross the bridge, now it's more like 160,000. We just don't know the precise reason for the problem."

The city, which is in charge of the drawbridge, is now moving the electrical system from conduits under the bascules to more flexible conduits in a more stable place, along the girders. This is expected to take another three to six months.

During that time, the bridge will be opened on a limited basis, using a temporary electrical system. The new hours for opening are as follows:

Monday through Friday -- from midnight to 4 a.m. for all boats, and at noon for pleasure craft and from 10 a.m. to 1 p.m. for commercial craft.

Saturday, Sunday and holidays -- from midnight to 6 a.m., and from noon to 9 p.m. for both pleasure and commercial boats.

Dr. Gridlock has been advocating some kind of commuter group that can provide an independent review of things that have gone wrong, affecting motorists. However, in this case, even though it is a shame to have to do electrical construction twice in two years, the restricted hours that the bridge can be opened helps motorists.

During the course of a year, about 58 million vehicles cross the bridge, while the number of times it is raised accommodates only 200 boats, according to Schoene. Each time the bridge is raised, which sometimes is for the benefit of a few people in a pleasure craft, about 8,000 people in vehicles are inconvenienced. The road is so heavily traveled, with more than 180,000 vehicles using it on a Friday, that sometimes the additional congestion caused by a bridge opening never clears up.

At one time, the 14th Street and Memorial bridges were draw spans, but the city decided cars took precedence over boat traffic and made them fixed spans. The Navy, among others, wants to keep the Wilson Bridge available for boat traffic. At a minimum, those involved in running the bridge might consider continuing the restricted hours even after the electrical system is fixed. Airport Annoyance

Dear Dr. Gridlock:

Who runs the short-term parking at National Airport across from American Airlines? Last weekend, for about the 10th time this year, I tried to use this lot to meet someone only to have it closed off with barriers. This time, there was even a policeman there to make sure no one tried to wait for the lot to open. The problem was that there were at least 10 open spaces in the lot, with more cars leaving.

There seems to be three lot attendants, or one worker and two friends. They are having a grand old time partying together, so I assume they are friends.

One likely explanation is that the workers close the lot so they won't be distracted from their conversations by anything so inconvenient as a customer. Why can't the police monitor the lot and make sure it is open when there are empty spaces. RAYMOND J. CARROLL College Station, Tex.

This is one of the busiest little parking lots around, handling 2,000 cars a day. Its central position in the circle in front of the main National Airport terminal makes it highly desirable for short-term use, so it's natural to want it to be run efficiently.

Normally the 122 spaces are controlled with a mechanical device that keeps track of capacity and puts up a red light at the entrance gate when the lot is full. Motorists must then wait at the light or circle around until someone leaves the lot. Earlier this summer, though, the mechanical system broke down, and had to be replaced by police officers using sawhorses, according to Dave Hess, a spokesman for the Metropolitan Washington Airports Authority. The mechanism supposedly has been fixed since you wrote in mid-July, Mr. Carroll. If anyone notes further problems, please let the doctor know.

Hess notes that there is another option, Short Term Lot B, on the main terminal road north of Lot A, just past the VIP spaces. Lot B has about 200 spaces.

Pains in the Butt

Dear Dr. Gridlock:

What, if anything, can be done about people who throw lighted cigarettes out of their vehicles? It is so frustrating to watch this occur many times a day. Do smokers have no concern whatever for the environment or care about the fire hazard they create? Is it possible to write down vehicle registration numbers and report them somewhere? CHRISTINE H. LAWSON Silver Spring

Police generally say they have to see traffic violations to cite people. Still, this is an annoyance. Why do smokers do it? Can't the cavalier flipping of lit cigarettes sometimes cause damage to other cars as well as to the natural surroundings? Just because we see it done constantly doesn't mean it is acceptable. Thoughts? Inattentive? No

Dear Dr. Gridlock:

This is in response to Peter Karpoff's letter {July 13} encouraging people to go immediately when traffic lights turn green. He accuses drivers of being inattentive. I guess he hasn't noticed that many motorists don't stop when traffic lights turn red.

Anyone who proceeds promptly when the light turns green without waiting a second or two to make sure no one is crashing the light is asking to be killed. This does impede traffic, especially on short left-turn arrow lights. But I would rather wait another cycle than get smashed by rude or negligent drivers who apparently are color-blind and don't know that red means "STOP." HERBERT A. ROSENTHAL Bethesda

Many lights have a delay of a couple of seconds where the intersection is red in all directions to let stragglers (and violators) clear, but given the amount of red-light running here, a quick, careful look in each direction seems prudent.

Dr. Gridlock appears in Metro 2 each Friday to explore what makes it difficult to get around on roads, from misleading signs to parking problems to chronic bottlenecks. We'll try to find out why bad situations exist and what is being done about them. You can suggest topics by writing (please don't phone) to DR. GRIDLOCK, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071. Please include your full name, address and day and evening phone numbers.