That does look very interesting indeed ... are there any mapping websites that allow you to automatically create something like this? WOuld only be a 1st estimate though, as local topography and traffic lights etc might dramatically change the distance you can cover in 15 minutes.
An interesting extension would be to do a 20/25/30 min map too, for larger places, like Aberdeen or Edinburgh ... and also to do a comparative map for cars ...
Many thanks HLaB!
T