To answer the question fully, bike paths in germany are mandatory to use when they have one of these signs attached to them:
it runs alongside a road (Strassenbegleitend) - meaning you are not required to follow it when it decides in midway it would rather lead to a different village.
Then there's the aspect of the "usability" of the bike path. Technically, a cyclist is not required zo use a bike path unfit to use. As this is subject to opinion, your's might vary from that of the policeman what bike path is usable and what isnt, so its up to the judges. (procedere as follows: Policeman tickets you, you appeal against it, matter goes to court). So far, accepted reasons for not using one have been glass splinters, snow, sudden ending in the middle of nowhere, no way of getting on it in the first place, and parked cars. The fact that you would have to ride at a very reduced speed due to bad condition of the lane is typically *not* a valid reason.
Unfortunately, no exception for racers in training exists. LJ is right, it has been proven by official counting that as a cyclist you have a better chance to be involved in an accident when you use them, between 3 and 9 times. (the police authorities draw the consequences out of that and stopped taking count).
On the positive side, most policemen will just send you on the lane and will not ticket you - but remember they could.
Also, don't be surprised when the typical german motorist tries to show you how dangerous it is what you're doing by putting you in more danger, like overtaking you very narrowly, or honking the horn wildly. If the first happens, notificate the fella to the authorities as this fulfills the crime of " endangering the traffic", which has hefty fines to it up to 1 year in jail.
If the latter happens, don't give the guy the finger, but act as if he was a dear lost friend and wave friendly. Not only is that cheaper, but also it annoys them no end
Despite all that, cycling in germany, as in all countries with a very dense population, can be very safe and fun, if you know which roads to use and which not to
Tire choice: Standard racing tires do it for most of us. No reason to yank out the knobby 32mm tires... this is neither Afghanistan, nor the cobbles of Ahrenberg...
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