Riding a bike is never 100% risk free. The degree of risk varies with the circumstances. Deciding whether it's safe enough or not is a matter of personal assessment of the degree of risk and risk tolerance. The speed of the traffic is just one factor in my personal assessment of risk. The road situation and traffic density figure in more strongly. I've ridden on interstates with traffic passing at 75 mph in a few places where it's legal (and maybe a few where it wasn't) and felt fine. I've been in situations where traffic was very dense moving ~30mph on 6 lane roads with lanes merging on and off the road and felt it was much more sketchy.