The "silent alarm," a radio signal that would summon help automatically in emergencies for bus driver, has been used successfully in both Atlanta and Syracuse, officials of transit systems in those cities said yesterday.

Washington's Metro officials promised to make the alarm system completely operational within the next two weeks as part of the settlement it reached with the bus drivers who staged a one-day walkout and snarled parts of the Metrobus system on Thursday.

For the alarm to be most effective, however, the driver must maintain his schedule carefully, despite traffic, weather and other obstacles. If the bus were off schedule, the police would have a harder time locating it in an emergency.

The alarm is part of a $5 million Metro program to install radios on all of its 1,800 buses. The radios have been installed and so have the buttons that activate the alarm. Computer programming must be completed in the next two weeks before the system will work.

Once pushed, the button sends out a radio signal that overrides all other communications flashes the word "emergency" and the four-digit identification number of the bus on a TV-type screen in Metro headquarters.

Five men sitting at consoles control the bus operation throughout the day. When the emergency sign flashes, the controllers type the bus number into their computers. The computer tells them instantly where the bus is supposed to be at that time, if it is on schedule. Police units would be sent to that location.

If the bus is not on schedule, the route has to be covered until it is found. Further, if the bus were hijacked and taken off the established route, the alarm would not pinpoint its location.

Just such an incident happened in Atlanta, where a quick-thinking bus driver gave the story a happy ending.

An armed man climbed aboard a bus of the Metropolitan Atlanta Rapid Transit Authority (MARTA) and hijacked it. The driver pushed the alarm.

MARTA bus controllers called the driver on the radio. "If I don't answer that, they'll know something is wrong and send the police," the driver told the hijacker.

The hijacker let him answer the radio call. The driver told central, "This man wants to be taken to the Georgia Capitol and we're on the way."

When the bus got there, MARTA spokesman Bob Brennan said, "about 9 million police were on the spot. The driver and a little old lady on the bus helped overpower the hijacker."

Atlanta has found the alarm effective in other situations, but "most of the time, the radio is all our drivers need," Brennan said.

"If we have a disturbance, the operators don't deal with it.They call for a supervisor, who is trained to deal with it. We can get a supervisor or a policeman to a bus within minutes."

Metro officials estimate that, with the alarm system funcioning properly, they can have police at Metrobuses in between 4 to 10 minutes.

That might not prevent the most violent crimes from happening, but "the only other alternative is a police state," said Nicholas Roll, Metro's assistant general manager for transit services.

Syracuse drivers also like the system, according to marketing director Al Babinicz. "We have used it very successfully and think it's a safety asset," he said.