You've had it happen to you. You're cruising down the Parkway, when all of a sudden traffic stops. Then, it begins to move at a snail's pace for an eternity.

All of a sudden, traffic is back up to speed, and you never know what happened.

It could very well have been a "slow down" conducted by New Jersey State Troopers.

The State Police has now released details of how and why they do these slowdowns:

When a Trooper conducts a slow down, he or she will slow down, activate the emergency lights, and drive across all lanes of the highway to prevent anyone from passing. There are various reasons for slow downs, but all have to do with safety. First, a slow down is sometimes used to eventually stop traffic altogether. This method allows for a slow, gradual stop, so that vehicles do not need suddenly brake, which could create accidents.

Other reasons could be that NJ DOT road crews are working ahead, other Troopers are removing debris from the roadway or broken down car, construction crews are closing a lane, etc. No matter what's happening down the road, if a vehicle passes the troop car conducting a slow down, that driver is most likely putting someone's life in grave danger.

So, should you ever encounter this in person, go slow and wait for the Trooper to stop weaving and drive away.