Max safe speed on a bicycle? Wind, how straight the road is, position on the bike, and cars limit that.
There is nothing like the feeling of wind whipping as your ride down a hill. I have almost been blown over several times. I have seen others blown over and only at a speed of about 30 mph on a light hill and these were some of the top amateur guys at an Ironman triathlon.
Couple decades ago, I did a race ,the first mountain we did(4000 ft climb)... guys were coming down the hill on their bicycles with their butts wedged between the top bar and the seat. And they flew by us as we were still riding up the mountain. (ya we were the caboose.) When it was our turn to go down the mountain... note this road was miles and miles of double digit grade 10% or higher and straight road, no switch backs, it's sucks to ride the breaks if you don't have to... at first I was reluctant to let go... but I joined in... A guy next to me was yelling out the speeds 50 mph, 55, 60, 65, 70, 75... I was scared... but we were all doing it, there was no wind and the road was closed off for us. 120 riders, and we were the slow ones! I can't image the speed at the front and their positions they were in to go faster... No body crashed going down that hill. So for that hill at that time 75 mph was reasonably safe.
Without wind you can fly down a hill on a bicycle if the road permits it. With wind, forget it... an accident waiting to happen at nearly any reasonable speed.