Geraint Jones alid
wrote:

I was coming down a hill at about 51km/hr, towards a roundabout ...

I don't understand: "51km/hr" doesn't seem to be a nice round speed in
imperial units. What sort of "about" was that, or do you have your speedo
calibrated in foreign and were you watching that when you should have been
not misjudging the line?

I glanced at it while descending the hill a moment or two before the
roundabout. It said 51km/h. I continued descending. Therefore I inferred
(from the available evidence, certain assumptions about the equipment,
and some basic principles of physics) that I was doing about 51km/h.

Daniele