NO STEREO read the black decal on...

NO STEREO read the black decal on the right rear window of the cream-colored Mercedes passing in the fast lane of the Santa Monica Freeway. Here was a clear sign of the way we talk to would-be criminals these days. The man in the Mercedes was saying, “Don’t bother to burglarize this car because it isn’t equipped with speakers, decks or other aural amenities.”

The bumper strip has been a means of political persuasion, celebrating candidates on the rear end of automobiles. The BABY ON BOARD signs have proved to be a controversial way to request caution from fellow travelers, claiming an innocent was enclosed. The “no stereo” message, by contrast, is a way of asking to be ignored--sent to common thugs rather than citizen voters or drivers.

Early in the century, when people wanted more protection than the police could provide, they put up warnings to the lawless: KEEP OUT or NO TRESPASSING.

Then, when warnings were not enough, when crime came to the nicest neighborhoods, people added threats and put up signs in front of private residences: PROTECTED BY INTRACTABLE SECURITY SYSTEMS. ARMED PATROL. GUARD DOGS. Even,VICIOUS GUARD DOGS. Benedict Canyon had a beauty in the 1960s, put up by a resident with a sense of humor to match a sense of fear: TRESPASSERS WILL BE VIOLATED. Or, maybe it was VIOLATORS WILL BE PERSECUTED.

Threats do not always work, so we have lately entered an era of discussion, obeying the Prophet Isaiah who said, “Come now, and let us reason together . . . though your sins be as scarlet, they shall be as white as snow.” The same logic--or hope--impels signs at service stations saying the likes of SAFE LOCKED. ATTENDANT HAS NO KEY. ATTENDANT HAS NO MORE THAN $50. The same sweet reason is behind signs in taxicabs saying DRIVER CARRIES NO MORE THAN $5 IN CHANGE.

Our neighborhood suffered a series of robberies not so long ago; up went the signs for security systems and armed patrols. But one resident resisted for a time, more worried about loss of life than loss of worldly goods. She despaired of preventing burglary but wanted to forestall robbery with a sign that would convey information rather than threats: “We’re sorry, we’re home right now. But all of us will be out of here by 8:30 a.m.” Finally, she joined the rest of the neighbors in calling the local patrol--as a better medium than her message.

All of us must hope the NO STEREOman is spared and that monaural motoring is the only price he pays for not being burgled. But suppose the thief wants a German ignition system, not a German sound system? A wary automobile owner might deliver a more effective communication by driving a 1968 Plymouth instead of a late-model Mercedes. Young car criminals, in fact, have been said to believe that the initials BMW stand for Break My Window--meaning the more opulent the container, the more likely a serious dent in the dialogue. The unseen but necessary sign of our times is Respect.