The city's largest transport union is taking on the mayor over the arrests of several bus drivers due to Vision Zero.

A new ad, paid for by the Transport Workers Union Local 100, shows Mayor Bill de Blasio handcuffing an MTA bus driver. It blasts the mayor for "criminalizing an entire class of hard working, law abiding unionized workers" during a "tragic right-of-way accident" where the driver was not at fault.

The mayor responded to the ad, saying it's misleading, and cited 21 fewer pedestrian traffic fatalities citywide this year.

The ad is set to start running in local newspapers Wednesday.