WILLIAMSBURG — When the MTA shuts down the L train and replaces service with shuttle buses and other alternative transit methods for 18 months starting in 2019, there could be so much Manhattan-bound road traffic that another bridge over the East River would be needed to fit it all, according to researchers.

It would take 1,154 bus trips a day with departures every 1.5 minutes if the MTA is to come anywhere close to offering a reasonable alternative to the subway line, according to maps released by data company CartoDB.

They show that if the Canarsie tube is shut down for repairs the fastest a shuttle bus would be able to get from Bedford Ave., the last stop in Brooklyn, into Manhattan is under 20 minutes.

The research found that anyone living past the Grand Street stop — the fourth Brooklyn station on the line — would be better off using either the A train if they're near Broadway Junction or the M train at Myrtle Avenue than a shuttle bus.

"There's no way city streets can handle that much traffic," CartoDB map scientist Andy Eschbacher said.

"If there are shuttle buses, [the trip] isn't likely to be 20 minutes."

In the apocalyptic post-L train scenario plotted out by the researchers, if the shuttle buses were to inevitably get stuck in rush-hour traffic between Brooklyn and the 1st Avenue L stop in Manhattan, straphangers would be better off ignoring the buses entirely and finding their way to another train line nearby. The M train or A train have some stops in the vicinity, depending on the section of Brooklyn.

"There would be major transit problems for people who can no longer take the L," Eschbacher said. "And there will be cascading effects for people who take other trains."

The map below shows the best alternate transit path to Manhattan for L train riders. Only residents in the red area will be able to get to Manhattan quickly.

Among those that will be hardest hit if the L train shutdown goes into effect are those who live in poverty.

The CartoDB maps show that 18,889 impoverished households live closer to the L line than any other train, including large sections of Brownsville, East New York and Bushwick.

"People that live in poverty don't have other transit options and they tend to work in jobs that are less flexible in their hours," warned Eschbacher.

The map below shows areas of high and low poverty along the L train.