I'm not sure that speeding on the whole is dangerous. If someone is on a quite none residential road and doing 35mph on a clear stretch this isn't so dangerous. It's people who insist on accelerating as hard as possible, braking late and hard, taking corners way to fast (often without indicating) who cause more of an issue even if they don't break the speed limit in doing so.
But, the whole point of the PE piece was....
"What Labour and Tory ministers have never understood is that speeding traffic is a problem because of intimidation, not just the occurrence of accidents. Over the past 40 years children’s independence of movement has reduced dramatically, with worrying health consequences, thanks largely to parental fear of traffic."