Here's urgent advice for motorists who might consider using Key Bridge to cross the Potomac River between Rosslyn and Georgetown: If you can, use another route.

An announcement at hand from the D.C. Public Works Department says repair work on the 62-year-old bridge will begin Monday, causing traffic delays. On a normal weekday, 72,000 vehicles use the crossing.

If work is to begin next week, then why were trucks and detour flagmen on the bridge earlier this week, causing nonrush-hour backups of traffic? Unwittingly, and without warning, I was one of hundreds caught in one such time-consuming jam. Apparently they don't count getting ready for repair work -- including cutting holes in the pavement of the bridge -- as part of the actual repair work.

Delayed drivers think it counts.

A large construction yard, ringed by a board fence, has been established at Rosslyn Circle. During the eight-month construction period, the normal three lanes on the bridge will remain open for Washington- bound traffic in the morning rush hours from 6 to 10 a.m. During this time, however, only one lane will be available for traffic going toward Virginia, promising huge backups reaching into the District.

At all other hours, including the afternoon rush period, two bridge lanes will be open in each direction.

Other restrictions will apply, notably affecting traffic from Washington headed toward Key Bridge. Watch the signs.

Warnings already have been posted on the George Washington Parkway in Virginia, west of Chain Bridge, urging Washington-bound traffic to use routes other than Key Bridge to reach the District. A Capitals Idea

If you're a Washington Capitals hockey fan looking for a way to attend next week's playoffs against the New York Rangers, Amtrak offers a deal you might not want to refuse.

For $145, if you're among the fortunate near the front of the line on Monday, the rail passenger company will sell a package of tickets good for a Metroliner trip to New York, a seat to either the Monday or Wednesday game and a sleeping car berth on the Amtrak "executive sleeper" returning overnight to Washington. A similar deal is available if there's another playoff game next Sunday. Amtrak has 40 tickets available for each game.

Madison Square Garden, where the games will be played, is atop Pennsylvania Station, Amtrak's New York terminus.

Tickets are available only at Amtrak's city ticket office, 1721 K St. NW, starting at 8:30 a.m. Monday. Telephone 383-3072. Wunder's X-Roads Update

Clifton G. Stoneburner, Arlington's retired highway and transportation chief, knows his old territory. And when he read in Metro Scene that the main intersection of Cherrydale was once called Wunder's X-Roads, he got quickly on the phone to dissent.

Wunder's X-Roads was actually the intersection of Lee Highway and North Glebe Road, a mile or so west of Cherrydale, Stoneburner said.

When established by Virginia highway officials in 1916, Stoneburner said, Lee Highway was the major route through what is now Arlington County. It extended from Rosslyn to the Tennessee border at Bristol. The northern half of the route is now designated U.S. 29.