I too can vouch for the "no loose items in the car"
I have unfortunately witnessed the result several times (once fataly)
Don't put anything in the car you wouldn't be injured by if it hits at 60mph (100kph)
Everything inside the car becomes a projectile and you are the softest squishiest thing in there
Always put your stuff in the Boot (Trunk) of the car - hopefuly you will never have to find out how true this is
This includes Camera Bags & most particularly Tripods
However, you also need to think about loose coins, thermos flasks and even hardback road atlases - non of these are fatal but can lead to nasty cuts and bruises.
Sorry for being so quite so cheerful