Joe and I set out to find out this very thing yesterday, and here are the findings.
Our methodology was to walk north for 5, 10 and 15 mins, then repeat for south, west and east.
Then using our cunning, we manages to figure out what kind of catchment area you end up with!
(Red=5mins or less, Orange = 10 or less and Yellow = 15 or less. Dah!)

We can use this data now to better understand who is in this space, what kind of distance away we should think about putting whatever it is we come up with etc. Well all kinds of things really.