This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
First, there are different classes of roads, delineated by the color of the
signs. For example, in Switzerland, the freeways use green signs (verboten for
bicycles), the blue signs are for primary roads(bikes OK) and the secondary
roads are in white (bikes OK). Primary roads tend to be a little more direct
than secondary roads, but they have more traffic as well.
The colors for road signs may differ from country to country. Note that in
France, freeway signs are in blue and primary road signs are in green.
One important sign to note is that in Europe, a red circle with a bike in the
center means that the road is closed to bicycles. In the US we are more
familiar with a red circle with a red slash through it meaning the activity in
the sign is prohibited, but in Europe, just the red circle means the activity i
the center is prohibited.
Many tunnels in Europe do not have lighting, and some are very long. For the
most part the road surfaces inside are OK, but it's best to play it safe and
slow way down, don't forget to pop up the sunglasses.
The mountain roads are generally good, but deteriorate as you go higher. Also,
the width of the roads can change dramatically from 2 lanes to 1 lane, etc.,
tunnels spring up out of nowhere, and the turns are not marked. However you
can avoid just about anything by being careful.
The roads in Switzerland, Austria and Liechtenstein are the best. France, Ital
and Spain are very good as well.