When is something with two wheels not a bike anymore? I imagine you would use a mass cutoff right? Or visual size? Bikes on the Tour de France get over 80mph on

When is something with two wheels not a bike anymore? I imagine you would use a mass cutoff right? Or visual size? Bikes on the Tour de France get over 80mph on down hill sections so making one "safe" at 60mph seems not impossible on first principles. Safe is usually judged by just a feeling and I would like to check out my feeling on such a machine. I'm sure this space of vehicle has not been worked on much and would love to see some active invention going on.