Earlier quoted context omitted.
if you're on a highway, then a car driving 30mph is much more dangerous than one which is 10mph over speed limit.
Do you have a statistic confirming that? Most highway deaths are due to speed, not due to driving slow. Driving slow can provoke people behind to do dangerous things, but it's on them, not on a slow driver.
So a car going 80 in a 60 hitting a car going 30 in a 60 is worse than a car going 80 in a 60 hitting a car going 60.
Now I'd rather everyone stay within a narrow limit and not go over the 'limit'.
Differential kills.