So what’s to be done about Downtown gridlock, now that Mayor Mike’s much-vaunted congestion-pricing scheme has slipped off to that giant traffic jam in the sky?

Answer: Send in the Marines.

Or at least the NYPD’s Internal Affairs Bureau, which last week started a major crackdown on illegally parked vehicles bearing official parking placards.

The offensive marks the newest wave of Mayor Bloomberg’s long-promised campaign against placard abuse, which was choking the streets of Lower Manhattan – and elsewhere – long before congestion pricing was cool.

As The Post first started reporting last year, use of the placards – which grant special parking perks to officials who allegedly need their cars for city business – has exploded in recent years, extending to thousands of employees who could have used mass transit.

Not surprisingly, so has abuse: Permit-holders (and counterfeiters) quickly found they could park nearly anywhere – for any purpose – with scant likelihood of getting ticketed.

The result: a congestion nightmare – especially in government-office-heavy Downtown.

But now Mayor Mike seems to be getting serious.

The IAB crackdown, which resulted in 150 summonses and 20 tow-aways of placard-bearing vehicles in only its first four days, follows his move to cut by 20 percent – or 14,000 – the 70,000 city-issued placards in circulation.

He’s also consolidated the formerly free-for-all distribution system in the hands of the Department of Transportation and the NYPD.

More remains to be done, of course – the crackdown needs to continue, and Hizzoner must stay firm with those non-city agencies that are fighting to keep their parking perks.

But it sure is refreshing to see the city face down congestion in a way that doesn’t involve a massive new tax on middle-class New Yorkers.

Indeed, this one’s a simple matter of City Hall getting its own house in order.

Keep on it, Mike.