There are more requirements for "safe" than "traffic free". In fact, there are arguments that 'safe' and 'traffic free' are semi-orthoganal, that is you can have safe conditions with traffic, and unsafe conditions without it. For instance, an unsurfaced road provides many ways in which a bicycle can easily be toppled - loose gravel/dirt, weird bumps caused by rocks, washouts, etc, hence probably not 'safe', at least compared to a smoothly paved surface which is kept obstacle free.