One evening a week ago, I saw a car parked on a shoulder of Interstate 395, near the Virginia end of the 14th Street bridge. A man standing outside it was videotaping planes on their approach to Reagan National Airport. It seemed like cause for suspicion.

I hoped to see a sign that had been advising motorists what number to call to report suspicious activity. Alas, the sign had been changed to warn me that I would pay a $100 fine if I was caught using my cell phone while driving in the District.

So I dialed the operator and asked for the D.C. Police Department. "Emergency or non-emergency?" I was asked. I explained what I had seen and was told to dial 411 and ask for the "non-emergency" number.

I did that and told the dispatcher what I had seen. He said that if I deemed the situation an emergency, I should hang up and dial 911, but in any case what I had witnessed was in Virginia.

By this time, I was in the heart of the District. I hung up and dialed 911.

"Montgomery County 911, what is your emergency?" I explained again that I was calling from the District on my cell phone and that the nearest tower must have routed my call to Montgomery. I added that what I had seen was actually in Arlington County. I was put on hold for 30 to 45 seconds before the dispatcher returned to give me a phone number in Arlington County. I dialed the 10-digit number.

A polite gentleman listened to my report and asked for my contact information. He then said that 15 minutes had passed since I had seen the suspicious activity and that a patrol car in the area, responding to a similar report, found that the video- grapher was gone.

I wonder if the other people who reported the man as he stood, illegally parked, videotaping planes flying low over the 14th Street bridge also had to dial four numbers before being able to report the suspicious activity.

I also wonder if, like me, they looked at the flashing signs on the bridge, hoping for a helpful phone number, only to be reminded that they shouldn't be calling anyone anyway.

CURTIS H. MARSHALL

Washington