Speed is one factor, difference in speed is a more useful measure of the risk of harm. If you're going 15mph on a road marked 35, you are in danger BECAUSE you are slower than the rest of the traffic on that road. Other vehicles expect traffic to go at or near the posted limit, if conditions allow it. Hitting a wall at 15mph is one hazard; getting hit by a car going 20mph faster than you is a bigger hazard.

In Hawaii, because some people tend to drive too slowly and doing so causes crashes, they post the minimum speed below the limit on a lot of roads. Across the US, it is illegal to go more than 15 below the limit in clear conditions.

Once a cyclist has a motor, it seems they are more likely to 'claim' a lane and then they need to keep up or risk being a roadblock. It's less safe either way, and I'm not sure there is a viable solution. Part of the thrill of cycling in traffic, I guess.