'Underground' radio: travel advisories and ads, ads, ads

In their relentless pursuit of an audience, advertisers have just found a few thousand more listeners -- stuck in tunnel traffic jams.

The Dewey Square (South Station) Tunnel in Boston recently became the third in the nation to treat motorists to a blend of traffic advisories and ads for everything from car rentals to horse racing. The innovative method: a radio signal that cuts in all across the AM band. Whether catching up with all-news, tapping along with country-Western, or relaxing to beautiful music -- every AM listener hears the low-power transmitter when the signal fades. The travelers are fed a few seconds or minutes, depending on traffic conditions, of underground messages.

The Boston tunnel provides ''near perfect'' conditions for tunnel radio, says Roger Skinner, founder of Tunnel Radio of America, Inc. (TRA), and the man who perfected the subterranean technology. The tunnel has a heavy traffic volume ( 154,000 cars per day) of local commuters. Cars take between 1 1/2 to 7 minutes to pass through the tunnel - plenty of time to run 30-or 60-second ads.

Mr. Skinner, acting as a franchiser, now is trying to win approval to install his system in New York City's Holland and Lincoln tunnels.

Other clients are negotiating with tunnel authorities in Denver, Norfolk, Va. , Pittsburgh, and Montreal. Looking overseas, TRA also held recent talks with the manager of the Cross-Harbor Tunnel in Hong Kong and this spring will scout out European tunnels.

Skinner first dug into the tunnel concept in 1974, while trying to get a license for a commercial radio station in Fort Lauderdale, Fla. Finding all available frequencies in use, he hunted for some other way to get on the air. Remembering the ''dead space'' he hit every time he went through the city's New River Tunnel, he decided to go underground.

After spending about $500,000 in 1975 to perfect his system, Skinner won special FCC approval to operate. He went on the air in the Fort Lauderdale tunnel in April 1976. Later, he put a system in a Baltimore tunnel for the Maryland Transit Authority.

The latest subterranean radio venture, Boston's Tunnel Radio Inc., sells 13 -week advertising packages for as low as $1-$2 per minute. It pays the Massachusetts Department of Public Works (DPW), which operates the tunnel, $500 per month plus 10 percent of any profits over $300,000.

So far, says Alan Radding, one of the principal owners, Tunnel Radio hasn't met with any opposition from local AM stations. One is even advertising on his system. ''We're preserving listeners for them,'' Mr. Radding argues. ''Drivers used to turn their radios off when they hit the static in the tunnel, and then forget to turn them back on. Now we hand (listeners) right back to the stations.''

For its part, the state DPW sees an opportunity to alert motorists to traffic and road conditions, says public information director William C. Pizzano. The tunnel awaits commuters near the end of the most heavily traveled route into the city, a roadway scheduled for major repairs over the next few years. For TRA's Skinner, a successful Boston operation could be good advertising. ''Other stations broadcast to a large area and make listeners search the dial to find them,'' he points out. ''With tunnel radio, listeners drive right through our station.''