It's not quite the L-pocalypse, but straphangers using the L Train nights and weekends may be in for a rough ride.

The MTA begins its long-awaited repairs to the line's East River tunnel Friday evening.

For 15 months, L Train service will be reduced every weeknight from 8 p.m. to 5 a.m. and every weekend so the repairs can be made.

The MTA says large crowds on platforms and lines to enter stations are possible.

 “I’m definitely expecting delays over 20 minutes, like 40 minutes to an hour,” one subway rider said.

"Chaos. But I guess a calculated chaos,” said another.

The MTA has put up signs, handed out fliers, and used social media to warn of the changes and to offer strategies so that riders can adapt.

But it's not clear everyone has gotten the message.

So, What's a Subway Rider to Do?

Your best bet is simply avoiding the L Train whenever possible by using other lines between Brooklyn and Manhattan. But MTA officials say there are other options for those of you who can’t.

“We want to make sure that our customers are aware that the beauty of the NYC subway system is that it is interconnected and it is redundant,” said Ronnie Hakim, the managing director of the MTA.

During the hours of tunnel work, L Trains will run only every 20 minutes between Manhattan and Lorimer Street in Brooklyn, and every 10 minutes between Lorimer Street and Canarsie.

But the MTA will increase service on the G, M and 7 lines to compensate. For example, trains will run every eight minutes on the G.

In Brooklyn, a new shuttle bus called the Williamsburg Link will be available to ferry riders between the Bedford and Lorimer stops on the L to nearby stations on the J, M and G lines.

And in Manhattan,14th Street bus service will be increased to shift riders to the J and M lines at Delancey and Essex Streets.

Transit advocates say riders should tweet any problems to the MTA and the city's Transportation Department

"Let the powers that be know how your commute is working, how your trip around the city is working, and what improvements can be made," said Danny Pearlstein, the policy director of Riders Alliance.

MTA officials say their staff will be out in force to manage crowds and make sure riders know all about their alternatives.