It's a bit unseemly, to be sure, but East Executive Avenue -- the narrow street that separates the White House from the Treasury -- is one of Washington's major commuter arteries. It's also one of the least dependable.

For weeks this year, the street was blockaded from traffic because sensitive underground communications cables into the White House were being reconstructed. Eventually, the street was reopened.

Last week, the blockades went up again. It wasn't for President Reagan's personal convenience, we're assured; he was out of town. Rather, the word is that the General Services Administration was installing new steam pipes into the executive mansion, and U.S. Park Police routinely erected barriers, forcing traffic to detour.

It's a good enough reason. But it's a source of much teeth-grinding and profanity among those heading into the McPherson Square (15th Street) area of downtown Washington who find themselves detoured instead around the White House into 17th Street and the traffic grid lock around Farragut Square.

Given the large number of times East Executive Avenue is closed, it seems worth asking budget boss David A. Stockman to approve a modest outlay for signs, to be erected as needed at and around 15th Street and Pennsylvania Avenue, proclaiming: East Executive Avenue Closed.

Our advice: Expect it to be closed tomorrow.