WE HATE TO keep returning to such a sore subject with motorists who must venture onto Northern Virginia's most confusing highways, but there's yet another trans-Potomac fuss over how to make the main lanes less than plain to navigators. This time we take you to Shirley Highway, where you may have noticed that there are reversible express lanes for rush-hour traffic. That's fine.

But what about these lanes at all other hours? Whether it's 10 o'clock in the morning or a quarter past midnight, you still aren't supposed to use the lanes unless you're in a car pool. Silly? Wait until you hear the objections of local officials to any easing of this weird restriction:

Virginia and District highway officials are arguing that -- and try to read along -- allowing just any old kind of traffic on these express lanes during off-hours would choke them up with traffic, without relieving congestion on the conventional lanes. Now, let's see -- you open up lanes and suddenly more cars appear at midday as well as midnight to add to whatever traffic is now using Shirley at those off-hours, right?

We'll find out, because a congressionally mandated one-year loosening of the restrictions -- to which these local officials are objecting -- is supposed to start early in '85. The change merely would open the express lanes to general traffic during non-rush hours. During the busy times, the lanes still would be restricted to four-member car pools.

Local officials are urging that the