I have wondered this myself. I can't see how there can be as quite frankly the speed you can cycle at on a road bike it would be dangerous to be on the cycle path! I can't see it being illegal.
Sven can you post a link to that law or tell me where in the BGB I could find it? I am very curious to read it as I cannot see how this is safe and also 'gesetzwidrig' to do it. A road cyclist can cycle much faster than those electronic push bike mini mofa things and they are allowed on the road.
Nobody said it makes sense! Studies even have shown, that spearate bike paths/lanes increase the chance of accidents with cars, as the car drivers do not pay attention to bikes on separate paths. As far as I know, in Berlin even the Police have officialy said, that they will not prosecute such an "offense". Sadly not in Frankfurt - Mr Sikorski, city official for traffic, wants to encourage people to ride bikes but rather wants to build more bike paths and so of coures wants people to use those.
Below I linked two pages in german about this. I actualy found out, that there no longer is a general rule to use cycle paths - according to Straßenverkehrsordnung, §2 Abs. 4., it is only obligatory if the paths are marked with one of the three signs at the top of the first linked page. But to me it seems, that this covers practicaly all cycle paths.
Radwegbenutzungspflicht at german Cyclists association
Radwege und Radwegbenutzungspflich at Verkehrslexikon.de
Edit: Concerning bicycle repair shops, there are two that should be all right in Ginnheim. I am not a bike enthusiast, so take my expertise with a grain of salt.
First there is the
Fahrradstation Ginnheim, which is almost right next to the U-Bahn-/Straßenbahn-Haltestelle Ginnheim. They opened only a few months ago but I have had very positive experiencs with them. Also, there is
Fahrrad Wagner in Alt-Ginnheim. They are a long standing bike shop - the only gripe I always had with them, is that they have their hands full practicaly all of the time. So if you want your bike repaired, this might take two or more weeks. Maybe this was just bad luck, don't know.