I'd like to create a visitor flow "topography" using a transportation layer (roads and trails) as a stream network and the administrative boundary as zero elevation or "sea level". The idea is to look at the "channels" exiting forest visitors take and to be able to identify what points are "downstream" of others along these routes. I'm doing this at regional and national scales, hence the need for such a model, instead of case-by-case scrutiny. Can I use create contours or a surface from just a "stream" layer (in this case a transportation layer) without elevations? Is there something else that already does what I'm trying to do?