Through evolution the human race has developed a body that can survive falling over at running speeds, or crashing into trees an rocks. We also notice that visual awareness of our surroundings starts to close in its focus as our speed of travel increases, and we also require assistance – road markings, advance warning signs etc, as speeds go up – this is why trams and light railways can operate ‘line of sight’ – with no complex signalling – but main line railways must have signals and other assistance to keep trains apart.

Perhaps than it is no surprise that as a long distance runner might manage 12-15mph and a sprinter break the 20mph barrier, a fit adult human can survive a ‘bump’ at up to 20mph, with relatively little risk of breaking bones (tests on human skulls in late1940’s estimated that a 20mph flat impact imposed just 30% of the load required for catastrophic collapse).

Thus 20mph is perhaps the limiting ‘design speed’ for moving people around.