Jim Burke <j.burke at earthlink.net> asked how to compute the intersection
between a set of lines (roads) and a set of polygons (regions).
This can be done using the spatstat package.
1. convert the polygons from the class SpatialPolygons (in the sp package)
to the class owin (in the spatstat package). See the vignette in the spatstat
package for details.
2. convert the lines (roads) to the class psp (in the spatstat package) similarly.
3. Use "[.psp" to clip the lines to the polygons. If X is your road network
and W is a polygon then X[W] is the road network clipped inside W.
Adrian Baddeley