When Mark Horowitz started riding his bicycle to work last April he sponged off the road dust in the men's room sink at Alexandria's City Hall. When gas began to vanish in May, the 35-year-old assistant city manager seized the opportunity to promote shower facilities.

Horowitz surveyed 600 city employes and found that 23 biked or jogged to work and 65 more would, if showers and lockers were available. Some said they'd use the fitness facilities on their lunch hours.

With the aid of an energy-conscious city council member, Horowitz prepared a proposal that the city spend $2,650 to install showers, lockers and a protected bike rack for employes. On June 12 the proposal passed unanimously.

"We found an old, unused bathroom and a storage room that will be perfect to convert to men's and women's locker rooms," he said. "The bid came in at $1,600, so we have $1,000 for extra benches and equipment."

One of the best ways to convince your cmpany to install similar facilities is to start a Fitness Commuters Club, says occupational health specialist Bert Knitter.Knitter is organizing a club at the Justice Department where employes have use of the old FBI exercise room.

Encourage membership of any employe who commutes by exercising, "whether on foot, roller skates, bicycle or pogo stick," says Knitter, who suggests posting notices on bulletin boards or wherever bicyclists lock their bikes.

"Make sure people know it's a viable alternative that can be economical, healthy and fun. People wonder what one person can do to stop the energy problem. This is a start."

Other ideas:

Boost membership by sponsoring seminars for prospective fitness commuters. Have experienced cyclists or joggers discuss such topics as what to wear and routes for jogging safety.

When the club is established, present your company's personnel or employe services director with your request for facilities. A company that won't install bike racks or showers for one individual might be impressed by a 50-member club.

Other ammunition Knitter says may help to persuade company officials to invest in "fitness commuters":

More than 200 American companies haves physical-fitness programs, including Exxon, Chase Manhattan and AT&T.

Minimum requirements for fitness commuters would be bike racks and separate changing rooms with showers for both men and women. Knitter says these installations would vary in cost according to the facility, but should be no more than $5,000.

"Fitness commuters are healthier, happier employes."

Eighteen bicycles can be parked in the space of one average-size car.

A company can "show employes it listens and responds to their concerns and show the public it's encouraging energy conservation."