MONTCLAIR, Calif. — The citrus groves are gone. Route 66, which once pulsed through this Los Angeles suburb, is a memory. Marilyn Monroe lives on mainly through tacky gift shop trinkets.

But another 1950s-era symbol of Southern California — the drive-in movie theater — continues to thrive here.

Mission Tiki, a Polynesian-themed drive-in with four screens and room for 1,000 cars, has been packed this summer, just as it was last year. At sunset on a Saturday in early July, with the San Gabriel Mountains glowing burnt orange in the distance, children with hot dogs ran between cars, a few teenagers on dates cozied behind dashboards, and families camped out in lawn chairs.

‘‘Got to get nachos before the new ‘Spider-Man’ starts,’’ said Maria Gonzalez, a 27-year-old sales clerk, following a popcorn-scented breeze toward the faux-bamboo snack bar.

Drive-ins remain a fragile business. Only 368 remain in the United States, and their numbers are dwindling by two or three a year, according to the United Drive-In Theatre Owners Association. But some survivors, like Mission Tiki, appear to be chugging along just fine — a few are even gaining steam — as a more affordable option to the multiplex, where ticket prices in a big city can run $12.50 or more for adults and $9.50 or more for children (not to mention an extra $3 or so for 3-D films that the drive-ins can’t show).

Mission Tiki, reflecting prices at drive-ins nationwide, charges $7 for adults and $1 for children ages 5 to 9. Younger children are admitted free.

“Don’t forget that you can also bring your own food to a drive-in, which is what we did,’’ said Kristy Dahlstrom, 23, a marketing assistant, as she sat in her Ford Focus with a friend at Mission Tiki awaiting the comedy ‘‘Magic Mike.’’ Picking up dinner at a McDonald’s drive-through on their way ‘‘got us an entire dinner for the price of a single popcorn at the regular theaters,’’ she said.

Drive-ins are where thriftiness and a fondness for the past converge with escapist entertainment — under the stars, no less.

Soupy cheese fries are part of the deal, footballs occasionally fly past the windows, and the romance in the next car can be more deserving of an R rating than the movie. And while the crackling speakers of old have been replaced by low-frequency radio transmissions, no one is expecting a state-of-the-art presentation — a good thing, considering that the headlights of late-arriving cars can end up co-starring with the characters on screen.

“Yep, we’re at the drive-in,’’ Dahlstrom said, as the lights of a sport utility vehicle momentarily merged with a shirtless Channing Tatum on the big screen.

There is no publicly available box-office data for drive-ins, but West Wind, a seven-theater chain centered in California, said 2011 ticket and concession revenue was up 43 percent compared with three years ago. Frank Huttinger, the chief executive of DeAnza Land and Leisure, which owns six drive-ins, including Mission Tiki and the Starlight Six in Atlanta, said his operation had ‘‘a substantial increase in attendance’’ this summer, compared with last.

Both West Wind and DeAnza are in the process of installing digital projection systems at their theaters, an investment that West Wind estimates at about $2 million.

“The conversion to digital is expensive, for sure, but it also means we think there is a future,’’ said John Vincent Jr., president of the drive-in association and owner of the Wellfleet Drive-In on Cape Cod.

Attendance there this summer is ‘‘extremely encouraging,’’ Vincent said.

The drive-in obituary has been written repeatedly. The first drive-in opened in 1933 in New Jersey, and about 4,000 were operating by the late 1950s. The solidification of daylight saving time in the 1960s contributed to the drive-in’s downfall, forcing later starting times. Widespread sales of color televisions hurt, too. Later, the VCR and multiplexes — called hardtops in movie theater parlance — took a toll.

And real estate development is a continuing threat. Drive-ins can occupy 30 acres or more, and originally were often built on low-value farmland. But urban sprawl eventually caught up, with shopping mall builders in particular buying and razing the outdoor theaters.

‘‘Every three years or so,’’ Huttinger said, ‘‘we are approached by developers at one of our various properties with an offer that we have to take seriously.’’