I tried working out the distance from Minneapolis to the Hudson bay using mapquest.com but I couldn't find a place with a road!

I think that I've worked out a methodology. Using a good map, use a piece of lightweight paper and a compass to work out the largest circle that only touches but does not ovelap the coastline at any point. The centre of the circle will be the furthest from the coast and concentric circles (like countour lines) will show places that are the same distance. I'm sure that there is an electronic equivalent, maybe copying a map into Powerpoint will do it.

Oh well, it's better than doing the accounts!