I've run a 42x16 single speed in a hard road ride. I draft going down hill, and don't sprint off the front going down. No problem at any speed over 15mph.
(Ok, I pedal doing 38mph down hill once, but that was about 180rpm, and not something I would want to do every day.)
Tight gearing does make a difference. I have tight gearing on my road wheel with slicks, and wider spaced gearing with my 40c tires.