Muni driver George Martensen was decorated for his attention to safety throughout his decades-long career. His renowned focus on the road likely blinded him to one of the most notorious stunts in San Francisco history happening behind his back.

On Sept. 30, 1969, Muni officials gathered in the Hall of Justice to watch a porno. It was filmed aboard one of their chartered buses while Martensen unknowingly drove the two performers from the Sunset into Golden Gate Park.

"What was going on," wrote the Chronicle reporter who watched alongside the stunned officials, "the movie showed with considerable imagination and cinemagraphic novelty – a man and woman copulating with singular industry on a vinyl seat."

The resulting half-hour movie was called “The Love Bus: Seat of Passion, Park of Pleasure.” It played in San Francisco’s adult theaters, was mocked in Herb Caen’s column and earned a place in Esquire’s 1969 Dubious Achievement Awards.

The Muni brass, one of whom called this the first porn he had ever watched, concluded that Martensen probably didn’t notice the ruckus behind him. Muni’s former Man of the Month was just that safe.

Not that his colleagues let him completely off the hook.

“George always was kidded about that filming and that ‘I didn't see a thing,’” said retired Muni official Angelo Figone, who as deputy general manager presented Martensen with a 32-year safe-driver award in 1985 in front of Mayor Dianne Feinstein. "We all suspected that exemplary operator Martensen had split vision: eyes on the road ahead and one in the rear-view mirror.”

The film’s director was a manager and cashier at the X-rated North Beach Movie named Terry Robinson. In June 1969 his small crew chartered the bus for four hours, without telling Muni why.

ALSO: This rogue Muni driver broke the rules and printed the first streetcar schedules

The “Love Bus” film itself is lost to porn history, and SFGATE couldn’t find Robinson for this story. As for what we do know, Robinson spliced scenes of the chartered bus with footage of a full one, giving the appearance that the mobile lovemaking was happening in front of a rush-hour crowd.

Michael Thomas, who worked for the film’s distributor, said he watched the movie once that year while patrolling the Presidio Theater as a manager during a midnight “Underground Cinema 12” showing.

“All kinds of cinematic flotsam and jetsam was assembled for these programs as long as there was a shock component or a flash of nudity at least,” said Thomas, who wasn’t involved with producing the “Love Bus.”

SF Chronicle

Thomas, now retired, credits Robinson with bringing “a unique lightness and humor” to the film but added, “I probably got bored when it got to the sex scenes.”

Police seized the “Love Bus” from the Presidio Theater when it premiered in the fall of 1969, as was common for any movie depicting sex at the time. But it went on to play alongside such other hits as “The Stewardesses 3D” and was advertised in the Chronicle movie listings. (The Presidio still exists and has long since switched to mainstream movies.)

The Chronicle’s Caen lent a heaping dose of attention and infamy when he wrote about the police seizing “that dirty movie,” and how a second reel was about to play at Les Natali’s North Beach theater.

“The cops already have the original print but I guess they can always use another,” Caen wrote. “As for the bus, Good Old No. 2603, it is having its seats cleaned.”

While the sex depicted on the bus was called “as passionate as it is gymnastic,” the film doesn’t show everything. Explicit sex would not be seen in a U.S. theater until four months later, when Alex De Renzy screened “Pornography in Denmark” in San Francisco.

As for what does appear in “Love Bus,” we can only go by the delightfully tongue-in-cheek Chronicle account 50 years ago.

The rollicking ride begins with Martensen picking up the young male lead onboard a 1955 Muni Mack bus. The actor ogles some female passengers until his blonde co-star boards the green-and-cream bus while licking a lollipop.

"That was extremely suggestive – set the tone for the whole movie,” said Muni’s Thomas Curran at his Hall of Justice review.

The man exchanges flirty looks over his newspaper until sealing the deal with, what else, a well-timed complaint about Muni as he tries in vain to help her shut her window.

"It sure will be nice when they decide to spend some money on some new buses,” he says. Proving that empathy is the best aphrodisiac, they rip off their clothes within seconds.

Martensen dutifully and safely drives this blooming romance into Golden Gate Park, where the couple disembark and "continue their tireless lovemaking on a blanket." She then runs naked into the trees to meet her husband, leaving the man stuck with a solo return trip.

Muni absolved Martensen from punishment – “He's a pretty reliable man. Very conscientious. Not the type,” Curran said. But it did require future charter bus passengers to guarantee in writing that their vehicles “won't be used for any immoral purpose.”

ALSO: You have to wait 42 percent longer or more for Muni than in 1949. Here's why.

Figone said Martensen retired around 1992, having never been involved in an accident, except the one that earned him national fame. One 1969 Chronicle letter writer from Virginia said the story “made my day.”

“For whatever it's worth, this expatriated/exported San Franciscan casts two votes for George Martensen for Muni Man of the Month (One for me and one for my wife.),” the letter says. “Keeping one's eyes on the traffic displays, without a doubt, a capability for silence and relaxation above and beyond the call of duty.”

Greg Keraghosian is an SFGATE homepage editor. Email: greg.keraghosian@sfgate.com