Proximity functionality in GRASS?

[grass at era.com]

Hi,
	How would GRASS best answer a question like:

"Show me all areas that are 5 miles from site A?"

				-OR-
"What areas are 50 meters from highway C AND have a slope of 3-5 degrees?"

Assuming I already know the terrain characteristics, what is the best method
for asking GRASS this question?  I was thinking of some sort of proximity
function, but I've found nothing in the manual so far.

Any suggestions would be greatly appreciated,
	Dianne