RARELY HAS SO little snow snarled so much and delayed so many. The snow-stick at Reagan National Airport Tuesday evening showed a total accumulation of 0.4 inches; at Dulles, half that. But the accumulation of inert vehicles--inside, outside and all around the Beltway--proved to be a showstopper. How Greater Washington can become so incapacitated by so few flakes is an age-old mystery, but Tuesday evening's mess offered a few clues: It isn't so much that too many people don't know how to drive in snow and ice; some just don't know how to drive thoughtfully or obey basic rules of traffic.

Downtown, far too many streets were blocked interminably by motorists who insisted on pulling into intersections instead of waiting for a green light and enough room to get fully across. In some cities, that's a major infraction, but here only a few intersections have been marked with warnings, and even at those, no officers were in sight to crack down. District officials acknowledged yesterday that too few police officers were dispatched to direct traffic--only 40 to 50 covered 50 intersections. Authorities should consider more use of trainees as well as additional shifts to guide motorists through jams where possible. And they shouldn't tolerate drivers who pull into intersections knowing they won't get across.

Granted, the forecast of a "dusting" didn't take into account the combination of other factors that produced treacherous road surfaces. The powdery snow stuck immediately to cold roads; and as traffic increased, the covering turned quickly into a slippery glaze. In town and in the suburbs, roads filled with traffic too swiftly for sand or salt trucks to let fly.

Metrorail riders generally had the best time of it, though even this system ran into problems. A Yellow Line train got stuck between the Pentagon and L'Enfant Plaza stations after a water pipe cracked and fell onto the third rail, causing delays on the Yellow, Green and Blue lines and extra crowding on some Orange Line trains. But without Metro, which carried 212,000 riders between 3:30 p.m. and 7:30 p.m., the streets would have been that much more intolerable. This morning again, Metro may be the best option for many commuters. For those who do drive, a supply of courtesy might help ease the way.