At 7:02 Wednesday morning, Cathy Alexander Houston got out of the car her roommate was driving, walked over the the shelter where the San Francisco commuter bus stops, and pulled from her purse a high-visibility yellow sign that said, in large black letters, "FD3."

At 7:04 Helen Petrichko parked her own car, crossed the street to the shelter, and pulled out a yellow sign that said "FD5."

They stood on the sidewalk in trench coats and printed scarves and black pumps, watching the traffic, holding up their checkbook-sized signs.

At 7:10, Nancy Worthington drove up in a silver and burgundy Dodge Diplomat with a yellow FD5 sign clipped to the sun visor. Petrichko got in. Houston poked her head in: Could they make a stop at FD3? No problem. Houston got to her contract analyst's job, Petrichko and Worthington got to their secretarial jobs, all three zipped through the commuter lane and the free car-poolers' tollgate, and nobody paid for the bus.

One is not to call it hitchhiking because that makes its organizers nervous. But this suburb of San Francisco has just inaugurated an intriguing - and in the United States, apparently unprecedented - alternative to gas lines - the block-long queues of idling anger that prompted California's new semi-rationing plan.

It also eliminates bus tickets, demanding car pools, wallet-searing parking fees and the various other splendors of rush hour commuting.

It is a licensed, informal system of ride pick-ups by which hundreds of commuters like Helen Petrichko (who would look a little odd standing there in her tan gloves with "Financial District" lettered on a shirt cardboard) report to a convenient gathering place - and then get a lift to work.

The flash-card commuters fit right in with the other means of transit in Marin County, which has come up with so many mildly hedonistic ways to get to work. It is now possible, for example, to ride your Gitane 10-speed to the ferry terminal, drink coffee and orange juice while gazing starboard across the water at the Golden Gate Bridge, and then board a cable car to the office.

Every participant in "Commuter Connection" pays a $5 registration fee for a burgundy plastic identification packet that contains a color head-shot photograph against a bright yellow background. Then the commuter picks out two destination symbols - black letter/number codes printed on transparent plastic. Cathy Houston, for example, goes from NO3 (an intersection depicted on the little map that comes with the packet) to FD3, at the edge of San Francisco's financial district. If she's riding, she just stands out there and flashes her card. If she's driving, she clips it to the sun visor and cruises by, looking for compatible NOs or FDs.

For 25 cents each, the connection sells extra transparencies: a rudimentary transfer system. Houston's roommate has a "GG" transparency which gets her as far as the San Francisco end of the Golden Gate Bridge, and from there she can proceed to any part of the financial district, or take a city bus off in another direction.

Connection officially opened on Tuesday, and for the time being, packets are being issued only to adults with standard identification (driver's license or military ID) and a San Francisco work or school address, which is checked by the organizers.

"Right now we're being very careful about it," says Ellen Eatough, the program's 29-year-old administrator. "It is strictly for commuting - and for people who live and work in the same community. We're not suggesting this for use on a dark and lonely road late at night."

State privacy laws forbid the connection people to examine criminal records, but Eatough believes the identification cards - "which allow us to plug into the police system" for quick investigation of any complaint - should keep the whole project safe.

Houston is not particularly worried. "Nothing's completely safe," she says. "Ever since I've started to drive in, I've stopped at bus stops and asked if people wanted a ride." She also doubts that there are many hard-core felons hanging around Novato at 7 in the morning. "You just have to make sure they don't fall asleep or something."

Eatough says the Connection was conceived about seven years ago, on the Golden Gate Bridge, in the front seat of a car that was staggering slowly, slowly through the same impossible traffic it was to stagger through at the end of the day. The driver, a commuting businessman apparently inspired by desperation, began making inquiries about possible support for a licensed hitchhiking program.

Eatough was brought in to take over, and last September, state and county transportation agencies finally came through with the $70,000 startup budget. That pays for advertising, a small Martin County office, two fulltime employes, and assorted part-time temporaries. If the registration fees are raised eventually to $15 or $20 a year, Eatough thinks, the Connection will be able to pay for itself.

Many questions are still unresolved: Does one pay the driver, and if so, how much? If the system expands how far should it go, and will it grow more difficult to keep track of more participants? Could it conceivably turn into a whole alternative transportation system and still maintain some assurance of safety?

"One step at a time," Eatough says. "Frankly, we didn't know the response would be on this wide a scale." Before opening day, about 200 Novato commuters had already signed up, and inquiries have been coming in from around the country.

In Virginia, Ken Stetten is watching the connection with interest. He is a technical consultant for an engineering firm with offices in McLean who for five years has been trying to interest Northern Virginia transportation officials in a licensed ride-sharing program he calls Community Auto Rapid Transit System (CARTS). It would separately license riders and drivers; the cars would carry little ID banners, maybe plastic CB antennae with one color-code for each destination point; the drivers would carry a flash card ID; and every rider would pay a fixed fee.

The California Connection has a grand kind of promise, as long as it stays safe, and there is an element of adventure infecting even some unlikely hitchhikers. One Martin County woman in her 60s caught a ride on the first day and reported back later, with some pride: "My first ride in a BMW." CAPTION: Picture, Helen Petrichko, left, and Cathy Houston, by Vici MacDonald for The Post