I did it last year. Coldest temperature was -18 degree C
the temperature is a problem because at that point if u breath too much it can damage your lungs..so slow driving is adviced. but not so much because of ice. just get some spike tires and its saver to go by bike then by foot because they really dig into the ice. should be legal in most countries since it is in germany. snow on the other hand really sucks.