A piece of Mayor Bill de Blasio's Vision Zero agenda that strengthened police's ability to punish motorists when a pedestrian is hit has been getting pushback from bus drivers' unions, as well as the City Council that passed the law.

The "right of way" law creates a misdemeanor charge for crashes that injure or kill pedestrians and bicyclists when the driver fails to "exercise due care." But support has grown in the City Council, including six members of the progressive caucus, to exempt MTA bus drivers.

Now, bus drivers' unions in support of the exemption from the Vision Zero law are emphasizing flaws with bus designs -- a large driver's side mirror and pillar between the windshield and side window -- that its officials say create a dangerous blind spot.

"You could see that the number of pedestrians that can be hidden behind them is enormous," said Brian Sherlock, safety director at the Amalgamated Transit Union and former Seattle bus driver. "You simply don't allow any structure to block view that can be engineered out."

Mark Henry, president of the union's Local 1056 representing Queens bus drivers and mechanics, said the MTA had brushed off concern about the design flaws, even before Vision Zero street safety policies were enacted.

"We've talked to the transit authority about it extensively," Henry said. The response, he said, was "make an adjustment and don't have an accident -- that's basically the rhetoric that we get."

MTA spokesman Kevin Ortiz said drivers are trained to get around the blind spots by "leaning into and out of their mirrors while seated," a method union officials felt was insufficient.

"Partial obstructions cannot be entirely avoided, but they can and must be effectively managed through seat adjustment and the effective use of mirrors on the bus," Ortiz said.

Paul Steely White, director of Transportation Alternatives and a street safety advocate, acknowledged that a blind spot can be a factor when a driver hits a pedestrians or bicyclists, but "that's no reason to throw driver accountability out the window."

Now, 25 council members back the unions' view that bus drivers should be exempt from the Vision Zero right of way law.

The main sponsor of the bus driver exemption, Councilman I. Daneek Miller, a former ATU Local 1056 chief and bus driver, voted for the Vision Zero law after discussing the "unintended consequences."

"No one knew that bus operators were going to be arrested," he said of his colleagues. "We really talked about criminalizing and vilifying public employees."

Wiley Norvell, a spokesman for de Blasio, said the mayor's "firmly opposed" to the bill.

At a news conference, the mayor defended the Vision Zero laws.

"Our message to everyone in this city, whether they work for the city or work for the MTA or they're a private individual, is you have to drive safely, you have to yield to pedestrians, you have to respect that there's new laws now that clearly penalize those who do not yield to pedestrians," he said.