"This is very much an auto and highway-dependent island," Mahmassani said. About 931,000 Puerto Ricans drive or carpool to work out of 3.4 million total residents, according to U.S. Census data. "There's just no public alternative that's really available."
The island's love of the car stems from urban sprawl that began in the 1940s -- and an equally long history of a public transportation system that was almost impossible to use, said Edwin Melendez, director of the Center for Puerto Rican Studies at Hunter College at the City University of New York.
For decades, the only public transit came from buses. "To say that they were unreliable is actually treating them too generously," said Melendez, an urban planning professor who lived on the island until he was 26. They were used by students who couldn't drive, by the old, by the poor and no one else, he said.
Puerto Rico's attempt to change that was the Tren Urbano, a 10.7-mile rail transit system that opened in 2004 and was intended to ease traffic between San Juan and nearby cities. It was a flop, in part because it didn't go places people wanted to go.
"The idea was that you would build it and they would come," Melendez said. "It was going to be this little island of urbanism, on the routes where they built this horrendous monster. It didn't happen. All of the predictions were wrong."
That helps explain why the island has the fifth-highest number of vehicles per capita in the world, after New Zealand, Brunei, Iceland and Monaco, according to data from the World Bank. Another measure: Puerto Rico has about 2.01 million cars, or roughly 57 vehicles per lane-mile of highway, compared with about 38 per lane-mile on the U.S. mainland, in part because the island has fewer miles of highway, period, Mahmassani said.
Melendez said the island's reliance on cars is not the only reason people are standing in line. He said his family is using gasoline to fuel a generator for an elderly relative who needs to keep cool. "They are using it for a lot of things," he said.
Getting gas for any purpose remains a challenge as fallen debris blocks roads, hampering deliveries.
Reporting for gas prices and availability has been sporadic on the island since the hurricane, with most gas stations offline and likely unable to fill tanks, said Dan McTeague, senior petroleum analyst at Gasbuddy.com.
"If you can't power your terminal, you can't dispense or move fuel," McTeague said.