Word of warning, alot of the tipper men around our way have them, and a couple of times it has been known for them to come in with such force that the driver side window has gone! Both times it was when both trucks had them on and being tarmac tippers wern`t exactly going slow.
Lycanthrope:
I thought these were only for tipper drivers ?
they’re very handy on livestock trucks too 
I got mine from HERE