[R-sig-Geo] cost distance buffer?

[Bastien.Ferland-Raymond at mrnf.gouv.qc.ca]

Hello R people

I want to create a buffer around shapelines that would go around obstacle like shown in that figure:

https://www.dropbox.com/s/8qhkj6atkfllzky/unreachable_buffer.jpg

Here is the idea: I have field teams that cannot walk more than 1.5 km away from a road.  I need to know what part of the territory is accessible and what part isn't.  I cannot do a simple buffer as the other side of the lake isn't really accessible by walking.  Anybody knows if it's possible to do in R?

It's been done a couple of years ago in ArcINFO but we can't know how because the code is hidden.  However it was using raster, so it's maybe a hint of a solution.

Any ideas would be greatly appreciated!
Thanks!

Bastien Ferland-Raymond, M.Sc. Stat., M.Sc. Biol.
Division des orientations et projets spéciaux
Direction des inventaires forestiers
Ministère des Ressources naturelles et de la Faune