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I've mentioned / requested things like this before, and there are two or three that I know of that do this, but this: http://www.mapmyfitness.com/maps/auto_route/ is the best implementation I've found for UK use at least. Also, that link takes you directly to the generator without needing to register. 🙂
Enter a start point and a distance and get a loop generated for you. Generally it needs a little checking / tweaking to make sure it hasn't taken you down towpaths or that type of thing, but very useful. I use it with http://ridewithgps.com to get a nice loop to transfer to the Garmin. Get involved.
Just tried it. All a-roads and no way to tell it to avoid them.
A lot of quite nasty A roads, but with a cheeky section of footpath that is currently pretty much unrideable!
EDIT: gave it a second go; while it's not illegal to cycle on the A14, it's not something I would choose to do....
Awful routes suggested here too
Heavily trafficked fast a roads - no thanks
Ok ok, try this instead - still the odd quirk of trying to take you down bridlepaths, but a more user friendly option!