Samuel J. Rosenberg noticed an ambulance parked at 20th and Q Streets NW a few days ago. Its engine was running. All its emergency lights were flashing. And Sam wonders whether this is consistent with current fuell conservation policies.

That's hard question to answer. Sometimes an ambulance engine is left running to power its flashing lights. The lights may be deemed necessary as a safety measure, or to make it easier for policemen who are en route to spot the scene of action. In hot weather, the engine may be kept running to power an ambulance's air conditioning unit, so that a sick person will not have to be transported in an overheated vehicle.

And sometimes, alas, an ambulance engine is left running for no better reason than that the operator isn't paying for its gasoline and therefore isn't concerned about wasting it. The passerby who happens to witness one fleeting moment in an ambulance's travels is simply not in a position to judge whether its engine should have been left running.

It happens that last week I had an experience that would have intrigued Sam. This column did not appear on Friday because I was sweating out some "minor operations" that were performed on my wife. (Everything except the patient's disposition turned out to be benign, thank you.) One day when I went out to lunch, I saw an ambulance outside the emergency room with its motor running, lights flashing and air conditioning unit apparently on. When I returned an hour later, the same ambulance was in the same position, still with everthing turned on. What I'll never know, Sam, is whether the ambulance had made one or more runs during that hour, or hadn't moved at all.