Bill de Blasio has hammered home a signature theme in the opening months of his mayoralty, and it goes like this:

He's the great commoner who feels our pain.

He's the beleaguered Brooklyn dad, the guy shoveling snow off the walk, the man who jumps out of his car to help city workers fill a pothole, the fretful fellow who just wants us all to drive safely and stay out of danger.

But last week he ran his own message off the road.

He got caught speeding. Not literally, but WCBS/2 did catch the mayor's motorcade roaring down the streets of Queens, ignoring the speed limit, blowing through stop signs and weaving from lane to lane.

No, that's not an impeachable offense.

But it's not good. Two days earlier, the mayor held a major news conference to unveil his Vision Zero plan, which aims to make New Yorkers safer by lowering the speed limit from 30 to 25 mph, assigning more police to traffic duty, and trying to bring vehicular deaths to zero.

It's a great idea. As de Blasio declared at its rollout, we must take "greater responsibility every time we get behind the wheel or step out on the street."

Unfortunately, Vision Zero is now coupled in the public mind with the viral footage of de Blasio racing through town with a royal entourage -- perched in the front seat of an SUV piloted by a member of his NYPD security team.

Bad optics, as the political consultants say. Worse, de Blasio tried to duck criticism by blaming the procedures of his security detail for the wild ride.

The misadventure is not just unflattering to the mayor, it's revealing. The NYPD answers to him. If he'd wanted his driver to obey the laws, he could have made it happen.

No mayor is totally consistent. Rudy Giuliani -- who reveled in his Beelzebub persona -- once tried to force a politeness campaign on Gotham.

But for sake of the city, we do hope de Blasio learns to mesh his carefully cultivated populist image with the hard reality that he sits at the controls of one of the world's most complicated places. A little leadership, please.