It's 9 o'clock at night. It's cold and rainy. You've been away from home for three days, this last one in long, dull meetings. The day seemed endless.

You're finally on the way to the airport to catch the last plane home to Washington. There's heavy traffic but you'll get there on time, you say to yourself, and you've got a confirmed reservation.

What you may not know is that 150 other people also hold "confirmed reservations" on that flight. The plane, however, has just 130 seats.

If all 150 prospective passengers show up, you may be one of the 130,000 persons - travelers with confirmed reservations like yours - who were denied boarding or "bumped" by the scheduled carriers last year.

It's all legal under current Civil Aeronautics Board regulations, but yesterday the board tol the airlines they must begin informing customers ahead of time that they have a chance - a "sight" one - that they may be bumped.

Starting April 3, all carriers must place "clearly visible and clearly readable" signs at each position at their ticket counters notifying customers of the possibility that fights may be "overbooked" and must attach notices with the same warning on every ticket sold.

In addition, travel agents must also post signs and enclose the ticket notices.

The required language of the notice informs passengers that those denied boarding may be entitled to a compensatory payment and that they can obtain information about the rules on payments at the ticket counter.

The regulations issued yesterday are "interim" pending a board decision in its overall reexamination of the carriers' overbooking practices.

The airlines have argued that overbooking - they sometimes refer to it as "space planning" - is necessary because many people make reservations, don't cancel them and don't show up. If the carriers didn't overbook a certain percentage of the seats, they argue, they would wind up flying planes with more empty seats.

They became worried about their liability in overbooking situations last year when the Supreme Court held, in a case filed by Ralph Nader against Allegheny Airlines, that a passenger denied space on a flight could go to court seeking damages for fraudulent misrepresentation based on the carrier's failure to give notice of overbooking practices.

After that many of the carriers filed petitions with the CAB seeking revisions in their tariffs - fares and rules - stating that all of the carriers' flights are subject to overbooking and that previously confirmed space may not be provided.

Under CAB rules, an airline must take a passenger with a confirmed reservation who is denied boarding to his destination within two hours after the original flight's arrival, or reimburse him up to $200 for the ticket in addition to letting him keep the orignial ticket and use it.