Unless the busways in place during the L train shutdown are exclusively for transit all day, every day, the transportation system will break down and Lower Manhattan will flood with Ubers, advocates warned today.

The Riders Alliance, Transportation Alternatives, and the Tri-State Transportation Campaign are calling on Mayor de Blasio to guarantee 24/7 bus lanes for the surface transit that will carry heavy loads when the western portion of the L train is out of commission.

The MTA expects tens of thousands of diverted L train riders to rely on bus services connecting Williamsburg and Manhattan. To keep buses moving, DOT will make 14th Street bus- and deliveries-only between Third Avenue and Eighth Avenue, and implement bus-priority lanes on the Williamsburg Bridge and the streets connecting to it.

So far, DOT has said the bus lanes will be reserved for transit during “peak hours” without defining exactly what that means. Some peak hour bus lanes in NYC are in effect as little as six hours a day — three for the morning rush and three for the evening.

But L train ridership remains high on nights and weekends. When the train is down for repairs, on 14th Street alone, DOT and the MTA expect more than 1,000 trips each hour from 6 a.m. to midnight, and buses will run at least every three minutes as early as 6 a.m. and as late as 10 p.m.

“The L train is a 24-hour-a-day train, the communities that rely on the L train are 24-hour-a-day communities, and the plan to accommodate L train riders has to be 24 hours a day as well,” said Riders Alliance Executive Director John Raskin. “People build their lives around the L train because they know that they can show up at any time of the day, step on the train, and get where they’re going. In order to be a true replacement for L train service during construction, buses need to behave the same way.”

Pushing in the opposite direction are a group of NIMBYs based in the West Village and led by attorney Arthur Schwartz. They’re suing DOT and the MTA to prevent implementation of busways and bikeways during the shutdown because they think their streets will be swamped with spillover traffic. But scrapping the busway would only exacerbate that problem.

“The worst possible outcome here is that those hundreds of thousands of L train commuters defect from public transit, seeking their own private car solution,” said TransAlt Executive Director Paul Steely White. “If the DOT and the MTA only bring a part-time busway, that will create ambiguity in the minds of commuters and motorists alike, and it will inundate local streets with more traffic.”