For more than a decade California has pushed automakers to green their lineups by selling zero-emission vehicles to aid its fight against pollution and greenhouse gases. But as more people opt for on-demand ride services rather than driving themselves, it was only a matter of time before state officials targeted Uber and Lyft fleets to reduce emissions, reports Forbes.

A bill wending its way through the State Senate would mandate that starting in 2023, 20% of miles traveled by ride-hailing services be in zero-emission vehicles (i.e., electrics), rising to 50% by 2026. By 2030, 100% of vehicles purchased, leased or contracted by the services would have to be ZEVs.  Dubbed E-CAr, or Electrify California Ride-hailing, the bill went to Senate Appropriations for review this week after clearing two other committees in April. If approved to move forward, it must then pass both houses of the legislature and be signed by the governor to become law.

“It makes the most sense to focus on those cars that are going to be on the road the most,” the bill’s author, state Sen. Nancy Skinner, told Forbes. “It doesn’t necessarily make sense to have all of our electric vehicles be somebody’s second or third vehicle that’s mostly just parked in their garage.”

Given the scale of Uber and Lyft’s networks in California, the most populous U.S. state, Skinner’s bill could have a major impact on the national electric vehicle market. In 2017 a total of 199,826 electric vehicles were sold nationwide, with California accounting for nearly half. Uber and Lyft currently have more than 200,0000 drivers on their platforms in the Golden State, suggesting a huge potential market.

For the E-CAr plan to work effectively, however, it will need incentives to encourage Uber, Lyft, automakers and drivers to switch from gasoline to batteries. That’s because EVs typically cost at least $10,000 more than equivalent gasoline-powered vehicles, a cost that has to be borne by the drivers, not the companies.

“The cars are not actually owned by Uber or Lyft, so the challenge is how do you operationalize those rules? That’s part of the problem,” said Dan Sperling, director of the Institute of Transportation Studies at the University of California, Davis and a member of the California Air Resources Board, which created and oversees the state’s ZEV program. For the legislation to work as intended, incentives are needed to encourage large-scale adoption of EVs by people driving for rideshare companies, including discounted lease and purchase programs from automakers, he said.

• California wants to make Uber & Lyft go electric.