A fat chap can easily go above 20 mph on a flat road.
It's almost impossible to judge speeds with the naked eye.
But as a mamil with some experience on the subject, like you I find it hard to believe he was travelling at anything even close to 20mph.
Hey geeks, one thing that has occurred to me watching again as this nimrod makes a play for King Nimrod - Lord of all Nimrods:-
Just imagine if that was the closest any of us got to a RTA.
What a truly amazing feeling that would be.