WITH MORE THAN usual ado, your local American Automobile Association has sounded a big warning about an all-too-little span over the Potomac. The message is worth reinforcing: beware (or better yet, avoid) the Cabin John Bridge during the day. Since reconstruction -- not the historic Reconstruction, but the repair work that began three weeks ago -- there have been five accidents on this bridge. The AAA is warning that motorists may be in for "a 16-month-long headache" until work is completed. Why should the work have to take that long?

For one thing, this is no mere pothole-patching job. The bridge has been in terrible shape for as long as most can remember, and it's now getting a $14.3 million overhaul, which includes a complete re-decking and the joining of twin spans into a single bridge. Because, fortunately, all lanes are being left open during rush hours, work can't proceed apace. Workers have about three months to sink some moorings before the elements close in. For the time being, this means one lane on both the inner and outer loops is being closed from 9 a.m. to 3 p.m. In December it will mean additional closings between 8 p.m. and 6 a.m.

Why so many accidents? No matter how many warnings are posted or how many signs noting that the speed limit is 45, says the AAA, drivers insist on going 65 until they hit the backsides of cars at the bridge. And with more than 129,000 vehicles crossing every day, chances are we haven't heard the last of the dangers yet.

What to do? Go somewhere else. The AAA can tell you where to get off, or around, complete with free alternate-route maps for anyone who sends a self-addressed envelope to AAA-Potomac, 8111 Gatehouse Road, Falls Church, Va. 222047. Even if some of the routes seem longer, they are safer -- and motorists should bear the alternate crossings gladly.