Dear Dr. Gridlock:

This is about emissions standards for trucks and buses: Two weekends ago, we were coming back to the District along Interstate 66 with all the other weekend traffic when we came to gridlock outside Manassas. We had to CREEP along behind a Maryland tour bus that was belching acrid, nauseating smoke so thick that it blotted out the taillights in front of us and everything around us.

It was a gas chamber on wheels from which we had no escape for more than half an hour.

Our 1982 Toyota failed its August emissions test because it had a hair too many hydrocarbons, which couldn't be noticed with the "naked" nose, yet this monolith is free to go where it pleases with its black curtains of smoke behind it. What gives? EDNA AARON Alexandria

You're entitled to be outraged, Ms. Aaron. What gives is a double standard, of sorts. Buses and trucks do not have to get emissions inspections and are rarely pulled over by police for violating pollution laws.

We drivers, of course, do have to have our automobiles pass emissions inspections or we can't renew our vehicle registrations.

Ironically, the pollution that seems to us to be the worst -- the black soot billowing out of some trucks and buses -- is considered less important to meeting clean-air standards than the exhaust from automobiles. That's why there seems to be a double standard in emissions controls.

As air pollution wizards explain it to the befuddled doctor, gasoline fueled vehicles (most cars) emit hydrocarbons, which, although not visible, are an element in creating ozone. The Washington-Baltimore area has had trouble meeting federal air control laws regarding ozone, and hence the stringent emission standards for cars.

Most buses and trucks use diesel fuel, and those vehicles emit highly visible particulates, but not so many hydrocarbons. The level of particulates in our air is not severe enough to require emissions control testing, and because air pollution control is directed at the most serious problems, as the specialists see it, there has been no need to impose controls on diesel burners.

Some people inside local bureaucracies are trying to change that, and California is moving in that direction. As as you point out, Ms. Aaron, bus fumes are sickening. They do have a health impact. If they obscure traffic, they can also be a safety hazard. The bureaucracies just haven't come to terms with that yet.

People are working to change the laws to include mandatory inspections of diesel-fueled vehicles, but they could use support from state legislators, or D.C. local officials. What you can do is write those officials to give them ammunition in the face of strong lobbying from interests trying to keep things the way they are.

There are laws in the District, Maryland and Virginia against any vehicle emitting pollution beyond a certain measurement, but in the Maryland and Virginia, at least, you almost have to be an air pollution expert to determine whether there has been a violation, and have that testimoney stand up in court. Because police are not experts, they rarely cite people.

State Police also do random checks of trucks for safety reasons, and take a lot of them out of service for faulty mechanics, such as brakes. But the trucks are not tested for pollution. That could easily be done with a device called an opacity meter, but someone has to care enough to include that in the testing.

The District does issue citations for "excessive" smoke from buses, cars or trucks, according to the city's traffic police commander, Capt. David Baker. So perhaps the problem is less apparent in the city.

What can be done? If you see an ongoing problem, and have access to a cellular telephone or a pay phone, state police and D.C. police will take a call and may come to the scene to investigate. If you report a problem to police after the fact, you seem to be out of luck in Maryland and the city. Virginia State Police will help you get a warrant against a violator, according to spokeswoman Lucy Caldwell, only if the pollution caused a safety hazard. Police are not interested in hearing from motorists who simply find the smoke unpleasant.

Complainants also can contact Maryland or Virginia pollution control authorities. Officials there say they will trace the tags and send a letter to the owner of that vehicle requesting that the vehicle be brought into compliance with state pollution laws. Although the letter is only a request, those cited often comply, with many calling the state back to report that they have had the problem corrected.

In Maryland the contact is Daniel Meszler, mobile sources control administrator, Maryland Air Management Administration, 2500 Broening Highway, Baltimore, Md. 21224;

301- 631-3270.

In Virginia contact John Bowden, Department of Air Pollution Control, Mobile Source Operations, 7240-D Telegraph Square Dr., Lorton, Va. 22079; 703-339-8553.

These agencies will trace only those vehicles registered in their states.

What else can we do? Other than contacting local lawmakers to get trucks and buses subjected to emissions control testing, one can also call the company that owns the vehicle. Some companies have telephone numbers on the trucks and buses, and some encourage telephone calls about driving habits.

The owner of the tour bus you cited, Ms. Aaron, said she was unaware of the problem but would have it checked and, if necessary, fixed.

Let us hope that the right people will step forward so that we may all breathe easier. D.C.'s 72-Hour Limit Dear Dr. Gridlock:

I recently received a $15 ticket for parking more than 72 hours in front of my own house. I walk to work and some weeks don't move the car between weekends.

I understand that the purpose of the law is to rid neighborhoods of abandoned cars. My car is 16 months old and looks like new. There were no old flyers under my windshield wipers.

How do the ticket issuers decide how long a car has been sitting in one place? JOHN H. WOLMELDORF Washington

It seems like you ought to be able to park in front of your own house indefinitely, but that is not the case in the District. D.C. law prohibits parking for more than 72 consecutive hours on a public street. The intent is to provide a basis for removing apparently abandoned vehicles.

However, the city almost never acts on this unless someone complains, according to public works spokeswoman Tara Hamilton. City ticket writers will then monitor the car for 72 hours.

If it appears abandoned, they will put a placard on the car to give notice to the owner that the car is subject to be towed after 72 hours. If the vehicle does not appear to be abandoned, no notice is given and the ticket writers simply check periodically to see if the car is in the same place. If it is, a ticket can be written.

Perhaps you might want to get together with your neighbors, assuming that is where the complaint came from, to see whether there is a way this can be worked out.

Dr. Gridlock will welcome your suggested New Year's resolutions for area officials. Suggestions should be simple, one or two sentences, if possible, and contain something you'd most like to see them do.

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.