The faster we bike, the fewer times we interact with motor vehicles so it's potentially safer to ride at 20 than it is at 15. Riding faster also causes less disruption to the traffic flow which reduces risk. If I could ride easily at 30 or 35 mph in traffic I absolutely would do so, and I think it would be much safer than riding 15-20.


At the extreme where we're fatigued, oxygen deprived and our coordination suffers due to maximal effort sure that will increase our risk of crashing but anything below that, not necessarily. You could say that reaction time is diminished at higher speeds, but we're talking about a span of several seconds, not tenths of seconds, so I believe that it is no particular factor either. In traffic that is, where the surface is mostly predictable and traffic mostly moves in predictable patterns. On a sidewalk or MUP, that's a different story.


So in short, the speed I'm going is not a big factor on whether I want to enjoy the no-helmet ride, because I don't see a whole lot of extra risk. It's more about ambient conditions, and where I'm riding.