The city and the MTA have a big blind spot in their L train shutdown plans: reliable bus service, advocates warned Wednesday.

Transportation agencies are underestimating the role buses will play when L train service to and through Manhattan shutters next April for 15 months, nonprofit Transportation Alternatives said in a report released Wednesday. Without improving the current plans, Manhattan will face a traffic armageddon, the group added.

“Our city has not yet grasped the enormity of this crisis,” said Paul Steely White, Transportation Alternatives’ executive director, at a news conference Wednesday afternoon. “It’s going to cause a lot of havoc for all New Yorkers unless the city musters an appropriate response.”

In December, the MTA and the city’s Department of Transportation released their joint strategy to deal with what the MTA considers to be the most impact-heavy service disruption in its history; the work, to deal with flooding damages from superstorm Sandy, will force 225,000 daily L train riders to find a new way to get between Manhattan and Brooklyn. The agencies are planning to launch at least three new shuttle bus routes between the boroughs — but they’re banking on 80 percent of those displaced subway riders to switch to other, nearby subway lines.

White thinks the agencies are “overestimating” the role of the subways, which he said cannot shoulder the additional burden — even as the MTA plans to boost service.

The Transportation Alternatives report, based on findings from the planning firm BRT Planning International, details five ways the agencies could ensure the buses run at subway-like efficiency.

At the moment, the MTA and the city are planning to create a busway across 14th Street, restricting private vehicle access to allow for steady and frequent bus service between Ninth and Third avenues during “peak hours” that have not yet been defined.

The report calls for an around-the-clock expansion of those restrictions and an extension of the busway to run from the Hudson to the East rivers. It also calls for more exclusive bus lanes in Brooklyn that would, in theory, help keep shuttles from getting snarled in the borough

Polly Trottenberg, the city’s Transportation commissioner, told reporters Wednesday evening at a community input session that she understood buses would need to be reliable, and that the agencies were working out how to tweak their plans based on feedback.

“You have to make sure the buses travel at a speed that’s fast enough that it attracts riders. That said, the mechanics of getting there … that’s a lot of challenges,” said Trottenberg,. “We are working through all that.”

Danny Pearlstein of the advocacy group Riders Alliance said buses will need to move at 12 miles per hour in order to effectively move commuters across 14th Street.