This morning I happened across a 2011 test service that Microsoft developed to automatically detect roads with Bing Aerial Imagery. 

Here is the blog article:

http://www.bing.com/community/site_blogs/b/maps/archive/2011/02/03/automatically-detect-roads-with-bing-aerial-imagery.aspx

There is a link to a short 30-second video of the process.

There is also a link to the service description, which describes the inputs and outputs with a sample application:

http://magicshop.cloudapp.net/

I tried it for the Missoula area.  Worked pretty well for well-defined urban streets without much shadow.   Did not work well for forest roads. Since Microsoft is really working to get new imagery out, this may be a good way to do some simple updates.