Direct mailing, the marketing technique long used in advertising and politics, is helping planners in Montgomery County's transportation department plot commuter habits and traffic trends years in advance.

With the help of Val-Pac of Maryland, the Bethesda outlet of a national mailing firm, the department's traffic control division has sent promotional flyers and computer card-sized questionnaires to 20,000 residents of the Olney and Potomac areas. The mailing was included in envelopes of discount coupons, according to Genevieve Leary, the traffic division's assistant chief.

"This was the first time we used this particular marketing medium, and the response has been good," Leary said of the Nov. 18 mailing. The flyers to Olney promoted the county's van-pool program; the Potomac cards asked commuters' opinions of Metro's rail and bus service, and also asked those who drive to work to rate rush-hour traveling.

Results from Potomac area survey -- 300 cards out of an expected 1,000 have been returned so far -- may help planners draw bus routes and control traffic when Metro opens eight Montgomery County rail stations along 11 miles of a 14-mile Red Line extension. The opening of the Friendship Heights, Bethesda, National Institutes of Health, Grosvenor, White Flint, Twinbrook, Rockville and Shady Grove stops is now scheduled for late 1984, and survey results may help planners decide whether to set up "feeder" buses between the stations.

The early returns showed that many commuters were dissatisfied with Metro's bus service and unhappy about current rail fares, Leary said after a cursory look at the replies. A computer will analyze the data later this month, she said.

The combined cost of the Olney and Potomac mailings was $700, a small price "if it can show where the buses will be the most needed and if it can target what routes will be most heavily traveled," said Barbara Lee, who manages Val-Pac's Bethesda office.

For 3 1/2 cents each, the flyers and survey cards were designed, printed and mailed at a Val-Pac center in St. Petersburg, Fla. Because the firm already had access to a mailing list containing the addresses of Montgomery County residents -- usually the costliest aspect of direct mail -- the county did not have to buy one, and saved hundreds of dollars, Lee said.