Every now and then I see department of transportation trucks on the freeway with a large box suspended from the back. It’s on a hinge, so sometimes it’s folded up, and sometimes it is hanging out over the road.
Is this a sensor array of some kind? What are they measuring?
It’s there to absorb the impact if they are hit from behind. Some of them are called energy absorbing units. They lower the box when they are moving slowly and might get hit from behind.
Bijou has it right, if it is the hinged box I am thinking of. They’re also usually covered in lights and reflectors to warn people.
If a work crew has to do something near an active lane of traffic, like cutting the grass, collecting litter, painting lines etc, then this vehicle follows behind them a few hundred yards behind at walking pace to act as a buffer to protect them.
Like this I assume:
http://www.highwaysmagazine.co.uk/filestore/King%20Trailers%20photo%20for%20page%20-%20Balfour%20Beatty%20Truck%20Sign%20Cushion.jpg
Apparently they are called “impact attenuator”.
It’s the same idea as the big sand or water filled barrels that are used at the front end of a guard rail.
Or more commonly, crash cushions.