How Wide Does A Wide Load Have To Be To Require a Police Escort?

Over vacation, I was rolling through Indiana and as I merged onto the interstate, I got passed by a bunch of police cars and a semi.

Now, I wasn't sure if I was seeing the transportation of some kind of top secret government thing, something just oversized, or if I had just found myself in the middle of some sort of real-life version of Speed.

Check this out - the truck is so wide it's taking up two full lanes of traffic, and it has cops all around it!