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.

You probably realize this but I just want to note that helmets and skulls have very different failure modes (by design in helmet's case) and just because your helmet split into pieces doesn't necessarily mean your skull would have as well.