ooo that is a nifty tool! not come across that before. I use a mixture of streetview, google earth and the garmin basecamp, as well as google maps. Thanks.
Any hints on tunnels in France? are there certain laws we need to be aware of?
If you click on a bit of cycle.travel route, there's a link for Streetview of the click location. For the UK, you also get a link for photos from geograph.org, which can be handy for checking the surface of tracks where streetview hasn't been.
Bikes may be banned in some of the longer tunnels. I'd take "will cycle.travel create a route through" as the easiest advance indicator - streetview may be old enough to predate the cycle ban (eg Bielsa tunnel, Pyrenees, 3 km @ 5%, where Streetview dates from 2009, and since 2011 it's alternating 1-way with traffic lights, so you'd have to get through in 10 min, no cycles aside).
Mostly, however, it's just a matter of obeying the "allumez vos feux" signs