A Tunlaw Road resident thinks that Metro ought to post warning signs in all its stations and on all its vehicles to alert passengers to the danger posed by pickpockets.

She says that twice in recent weeks she has left crowded buses and discovered that her purse had been opened and her wallet taken.

She adds, "Metro might take a lesson from London subway stations, which have prominently displayed signs that warn passengers to protect their belongings because pickpockets love crowds."

I'm not sure whether signs help or not. Reno Road is the most heavily signed street in town, with warnings about speed limits and strict enforcement posted every few feet. Drivers exceed the speed limit anyhow, perhaps because too many signs -- more signs than anybody can safely read while he's driving on such a narrow, twisting and busy roadway -- eventually result in a "visual clutter" in which the passerby sees nothing.

I realize that the written word can have great impact, but two questions arise. Question 1: Even when signs are noticed, are they necessarily obeyed, either by drivers on Reno Road or by passengers in Metro vehicles? Question 2: What better warning could this reader have had than being robbed the first time? Yet even that didn't make her more careful. After all, there are many things a woman can do to protect her purse when she enters a crowded environment.

Besides, some people who see a sign that warns them about pickpockets foolishly pat the pocket in which they carry their money. That makes life even easier for the crooks. ANNOUNCEMENTS REVISITED

So long as we are on the subject of Metro, let me tell you about reaction to the recent suggestion that subway operators should announce on which side of the train the doors will open.

Comments have been numerous, vociferous and enlightening. Let me give you a few samples:

Wayne Kaiser said subway announcements "aren't loud enough to hear about half the time." Richard Reiff said most announcements are unneeded; regular passengers know where the exits are and tourists can't identify the names of the streets that are being mentioned. Robert F. Bluthardt called the announcements about door openings "frivolous additions to an already abused address system."

J. W. Higgins made two perceptive comments. The first was: "Half the seats in each car face the rear. For passenger sitting in these seats, the announcements can be confusing." The second was: "Some Metro operators do not close the window beside them while the train is moving. Any announcement they make is almost unintelligible due to the increased background noise."

"He's right on both counts," said Metro's Cody Pfanstiehl. "We keep telling the operators that nobody can understand them when a window is open, but as you know, some people have to be told repeatedly before they get the message."

He added, "We were aware from the start that some passengers would be in seats that face the rear, and that when we'd say a door would open on thr right it would be on their left. So we looked for a brief but effective way to remind those facing the rear that left can be right and vice versa, but we haven't found it yet."

I commented that most people probably assume that a mention of "right" or "left" refers to a passenger facing in the direction in which the train is moving. Cody agreed that most people do understand this, but said Metro would like to eliminate all chance for confusion if it can.

When I asked him why announcements must be so short, he sighed.

"Occasionally," he said, "an operator will elect to say something like, 'Silver Spring station. The time is 5:42,' and we'll get a complaint from some guy who'll say, 'Stop yammering about what time it is. If I want to know the time, I can look at my watch.'"

Trying to cater to customers like that must be an awful pain in the neck.About all a polite Metro employee could say in response would be, "Thank you for sharing your views with us, sir -- and have a nice day anyhow." " THESE MODERN TIMES

Overheard in the National Press Club bar: "As a hedge against inflation, my wife has decided to spend all my money."