from my limited understanding of fixies, it’s not that they’re “dangerous”, just different, especially to a rider used to a freewheel bike
on a freewheel bike (SS and Gearies) the rear wheel can turn and the pedals do not turn, i.e., you can coast and backpedal
on a fixie, whenever the rear wheel is in motion, the pedals are moving, there is no way to coast on a fixie
if a rider used to a freewheel rides a fixie and attempts to coast, the pedals may lift him off the seat, worst case scenario, he might crash, until he’s comfortable with emergency maneuvers on a fixie, he should stay out of traffic for his own safety
many SS’ers and Fixie riders equip their bikes with a “flip-flop” rear wheel, it has a fixed gear cog on one side of the hub, and a freewheel hub on the other, so they can ride what feels more comfortable, a fixie on the flats/in races, and a freewheel on hilly roads that require coasting down hills
i’m still a newbie at the whole SS/Fixie thing myself, so i could be wrong…