To tell folks to get out of the way.
Seriously doubt its nukes, more likely some type of conventional missiles. Convoys with cargo of that nature stop as little as possible and stoplights dont mean a thing.
I was an armed guard in an escort vehicle moving conventional missiles from Ustka, Poland back to the ASP outside Ramstein, Germany in ~2012 and our convoy looked much the same.
If that had been something nuke weapons related, it would have been in a white over the road tractor-trailer with a couple no-descript SUVs around.
On a very well planned out path.
I was driving through middle of nowhere eastern WY / Western Nebraska a while back. There were HMMWV with mounted 50s at each intersection (think 2 rural roads crossing hundred sq miles of corn fields...), fleets of black suburbans flying around and a couple helicopters. When I finally got to my client they just said, "yeah...probably a nuke getting moved around".
Malmstrom AFB said it was likely a convoy associated with the base indicating they regularly have escorted convoys with ācomponentsā of the Minuteman III ICBMs