What took cops so long during biker melee?

How could it happen? In a city that prides itself on public safety, how could a pack of motorcyclists on the Henry Hudson Parkway set upon an SUV driver who simply ventured onto the roadway with them?

After the end of a terrifying chase lasting more than seven minutes, the driver, Alexian Lien, was dragged from his Range Rover and beaten and slashed in front of his wife and their 2-year-old child. A biker hit by Lien’s SUV as the family tried to flee the mob has a cracked spine and ribs.

Where were the cops?

The NYPD says the initial call from the SUV went out as a harassment incident and that police were faced with a moving crime scene as the vehicle and its pursuers roared up the parkway before finally halting at 178th Street and St. Nicholas Avenue, where Lien was attacked.

That’s not an acceptable answer.

The NYPD was well aware of the Sept. 29 motorcycle event known as Hollywood’s Block Party 2013. Officers set up checkpoints at places like the Brooklyn and George Washington bridges and confiscated 55 bikes for assorted infractions. On the plus side, they did prevent the bikers from bringing Times Square to a standstill, as Hollywood’s Block Party 2012 had done.

But this year, despite the rapidly growing pack totaling as many as 400 motorcycles by one estimate, the NYPD seems to have given little thought to how this throng could safely navigate city streets and highways.

When Lien drove his Range Rover onto the highway, some bikers say they just wanted to box him in so other motorcyclists could safely pass. A video that later went viral makes the bikers look far more like an ugly mob than traffic directors. That’s the way the Lien family saw it. And that’s the way the incident turned out.

An undercover NYPD detective saw the attack on the SUV, but it appears he didn’t do much. The off-duty cop was riding with the herd. Could he have alerted the NYPD, and why did he take so long to come forward?

The NYPD needs to review its tactics. The next time bikers descend on the city, it has to be ready.