California is at the vanguard of America’s transition away from a carbon-based economy. But the state risks taking a major step backward if its voters approve Proposition 6, which would roll back a recent gas tax increase and, with it, a major source of transportation funding. The ballot initiative would give Californians slightly lower prices at the pump, but only by sacrificing the sustainable mass transit we need if we’re going to stop cooking the planet.

If Prop 6 becomes law, and officials in other states decide to follow California’s lead, it would be a blow to sustainability at the worst possible moment. Last week, the United Nations Intergovernmental Panel on Climate Change projected that the world has only 12 years to overhaul the global economy if we’re to avoid cataclysm; we simply can’t afford unforced errors in the time we have left.

Voting has already begun on Prop 6, thanks to vote-by-mail laws. The uncertainty over the measure’s fate points to a contradiction at the heart of California’s climate policy. This summer, California’s Air Resources Board reported that the state had achieved its 2020 emissions reduction targets four years ahead of schedule. But the same report noted that transportation emissions rose slightly in 2016. The main culprit was “gasoline used in on-road vehicles.”

In other words, the state leading America’s green revolution is still addicted to cars. And its voters have been avoiding a decision that will define our future: whether to let go of a car culture dependent on cheap gas in exchange for a habitable climate. With Prop 6, it is time to finally choose.