An upcoming change to state environmental rules might make it possible to speed up the frustratingly slow Expo Line.

The change could set the stage for the city to give trains the right of way at some intersections—instead of cars.

Giving cars the right of way has been one of the biggest challenges to making the light rail, which runs from Downtown Los Angeles to Santa Monica, faster. Instead of flying over or tunneling under intersections, especially between Western Avenue and Downtown, the train stops and waits for traffic to pass in front of it.

At-grade crossings not only make for slow and jolty start-stop service, they can be dangerous. Inattentive drivers can turn in front of Metro trains, causing injuries, delays, and deaths.

But existing state law considers even a small delay to those cars to be a “negative environmental impact” that transit planners have to “mitigate.”

By January of 2020, however, state officials aim to complete a subtle—but important—revision to the California Environmental Quality Act. That revision will change how the state studies the traffic impacts caused by construction projects, including new transit lines.

It will stop using a metric called “level of service” and will start using a metric called “vehicle miles traveled.” The new metric will give preference to projects that reduce how many people drive in its study area.

Santa Monica City Councilmember Pam O’Connor says that in 1970, when CEQA first became law, the existing metric was interpreted as a way to study potential effects on air quality.

“Cars sitting in traffic and not moving were spewing out toxic fumes and other pollutants,” she says. “Since 1970, cars have gotten much, much cleaner. As time goes on, and we get even cleaner cars that are not spewing fumes like they once did, ‘level of service’ no longer functions as a proxy for air quality, because the cars aren’t polluting like they used to.”

Chris Ganson, a senior planner with the governor’s office of planning and research who’s working on the rule changes, says shifting to a vehicle miles traveled metric from a level of service measure means builders will be less constricted by a project’s potential impact on traffic.

“Giving a transit vehicle signal priority or preemption would not lead to more car travel,” he says. “It would likely mean less vehicle travel because it makes the transit mode a little more appealing relative to autos.”

Ideally, the Expo Line would have been built with more grade separation than it was. In the early 2000s, when the Expo Line was first being planned, Metro planners studied grade separation at intersections like Vermont, Normandie, and Western avenues, and Crenshaw Boulevard.

But separating the train at all of these intersections would have increased the cost of building the Expo Line, potentially beyond the point Metro could afford. That’s how we ended up with a train that runs at-grade on some stretches and above ground on others.

“The Expo Line is a great example of how evaluating transit based on impacts to car traffic can be counterproductive,” says Carter Rubin, a mobility and climate advocate at the Natural Resources Defense Council, who worked on transportation policies for Los Angeles Mayor Eric Garcetti’s office.

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Rubin points to Expo’s Overland Avenue crossing as an example. That intersection has crossing arms that automatically come down when a train approaches, giving it the right-of-way, but required environmental mitigation meant the street had to be widened from four lanes to six lanes to minimize car delays at the intersection.

“Overland Avenue had to be widened to make sure cars wouldn’t be delayed, but the Expo Line is designed to connect walkable communities with transit—not make it harder to cross the street,” he says.

Experts say that the city of Los Angeles, the jurisdiction that governs most of the Expo Line’s grade-level crossings, will likely switch its traffic analysis policy at some point in 2018. Once that happens, Metro will be in a stronger position to reevaluate how the Expo Line, as well as any of its transit lines with grade level crossings, pass through those intersections.

Still, the change won’t be easy to make. The Los Angeles Department of Transportation, which controls LA’s traffic signals, has worked with Metro to make trains flow as quickly as possible under existing law. But the argument against giving trains full priority is that doing so would likely make car traffic at least slightly worse near those intersections.

Los Angeles leaders have emphasized that the city needs to wean itself of private cars and more fully embrace public transit. Giving trains full signal preemption would be a step toward that goal.