Without a car, Carla Martinez has to plan her shopping trips in Petaluma carefully.

"It gets really crowded at school times," she said. "But I'm just glad to be able to use the bus. It's pretty convenient."

Headed home this week lugging two bags of groceries, Martinez was able to catch a Petaluma Transit bus not far from Grocery Outlet at a busy bus stop on Copeland Street.

Martinez, who has used the bus system for years, has noticed an increase in the number of passengers recently.

Her perception is reality. Ridership has more than doubled in the past five years, said Transit Manager Joe Rye.

The increase is so marked, in fact, that Petaluma recently bought three used buses from Santa Rosa to add to its fleet of eight buses.

At 40 feet, the 1999 New Flyers with more than half-a-million miles already are bigger than any of the current city buses and will hold Petaluma over for another two years, when it can buy new buses to augment the fleet.

Ridership has increased from about 150,000 a few years ago to about 350,000 last year, much of that attributed to a focus on serving junior high and high school students. About 1,600 people ride city buses each day, several hundred of whom are students.

Partnering with Petaluma City Schools, marketing bus passes to students and rescheduling bus routes to match school schedules helped immensely, said Transit Advisory Committee Chair Dave Alden.

"Convenience and dependability," he said. "When SMART comes into play, that will be even more important. You'll be able to know you can get off the bus and catch the SMART train."

Petaluma bought the retired Santa Rosa buses for the rock bottom price of $25,000 total. It will cost more to repaint and rebrand the buses than it did to buy them, Rye said.

"They're about 15 years old. They are definitely no spring chickens," he said. "But these are good buses."

He said they could last another four or five years, when they will then be used as replacement coaches.

Petaluma Transit, like most Bay Area transit agencies, buys its new replacement buses through the Metropolitan Transportation Commission's disbursement program, which provides 80 percent federal funding for bus purchases, but on a rigid timeline. Petaluma Transit isn't due for new buses in 2015 or 2016.

In the meantime, the three used buses, once they are outfitted and in use by April, will ease the standing-room-only crowd that often accumulates on 30- and 35-foot buses to and from Casa Grande High School, Petaluma Junior High School and Kenilworth Junior High School.

The city also is working to install free WiFi on city buses and is considering buying devices that would give buses longer green lights when approaching traffic signals.

Kenilworth Principal Emily Dunnagan said the city's help with bus service has been remarkable. At least 200 students ride city buses each day.

"They've been so supportive," she said. "A couple years ago, the school district had to cut the bus service to quite a few areas. City transit just jumped in, and it has been a godsend for Kenilworth students."