The law tends to address this in various ways. I don't know how German law does it, but usually there is a fairly exact description of what stopping for a traffic signal entails (e.g., the driver has to be present in the vehicle). Also most jurisdictions have a catch-all law that prohibits dangerous behaviour in traffic.

So yes, there probably is a legal obligation to keep moving. If you went there and stood in front of the light for an hour a friendly but stern police officer will — legally, and within his mandate — instruct you to move along.