While calculating kinetic energy is fun to do, it's not really applicable to personal safety in this case. If I'm going 60mph, then I'm most likely going forward. If I'm going forward and the remote head is on the dash, then the head unit will tend to go towards the front of the car in a crash, NOT towards me or my passengers. Personal safety is not an issue here. Yes, it may fly off and smash against the windshield, getting destroyed in the process, but that's just a financial loss, not bodily.
That said, take a look at the great mounts put out by pro-fit: http://www.pro-fit-intl.com/
They have a nice line of reasonably priced, well-made mounts. Vehicle specific, universal, window mounts, etc. I spoke with one of their mount designers and it turns out he's a ham and is trying to get the company to market to the amateur radio operators more.
I'm using the Ulti-mount for the head unit of my FT-8900, putting right on the dash so it's a short look down from the road to see it. It also has a swivel ball mount so when my wife's driving, I can turn it towards me in the passenger seat.
For my 706mkii, which I just bought, I think I'm going with "THe Bug" a new product they have. Both of those products use 3M auto-adhesive tape to stick to the car and this is easily removed if you want to, without residue, yet holds very tightly.
They have a line of "vehicle specific mounts" which will allow you to install without damaging your vehicle but for my grand caravan, it just put the head unit too far away for me. It might work great for others.
To see my mount for the FT-8900, you can go to this site: http://www.pbase.com/lehnherr/ham_radio