Actual stats from Great Britain in 2008 on motorways, presumably always speed limit above 50 MPH, "6% of the total killed, 3% of total seriously injured, 5% of total with slight injuries." at least per wikipedia.
So there's about a 9 in 10 chance if you crash on a GB motorway, at least in 2008, you'll walk away without a scratch, rather than your 1 in 5 stat.
Also all minor aircraft accidents involve flying faster than 50 MPH although death from anything but fire is relatively rare. True though, that controlled flight into terrain at cruise speed doesn't usually leave many large pieces on the mountainside.
You are confusing the speed limit with the speed at which the crash happens. Most people apply breaks before the crash, and do not crash at full speed.
Car accidents at 50 MPH result in 80% death.
760 MPH is much much much more, considering the amount of energy needed to be dispersed grows quadratically (E = 0.5 * m * v^2)