So are you trying to make the case that it is safe to road ride without a helmet? If so then I would say that ignoring the math is far less perilous than believing it! The risk may be lower than for mountain biking but it is certainly not zero. What are the downsides of wearing a helmet on the road? None as far as I am concerned.
i'm ambivalent about it
i think that helmets, on a number of counts, depress rates of cycling activity - the "but it will mess up my hair" issue is a canard; and whatever we may think of people who don't/won't ride because they have to wear a helmet isn't really important. helmts definitely don't reduce collision rates - and while they (marginally) reduce rates of head injuries among those who suffer collisions, they don't reduce rates of serious injury and hospitalization overall
increase cycling rates do impact safety, unequivocally, by reducing collision rates - see my second slide on the previous page
my attitude towards helmets is that personally i think they're a good idea, and i almost always wear one. for other people, i say wear one (on the road) if it makes you feel better - but if you're not going to ride if you have to wear one, then do ride and don't wear one, definitely: at both the public and individual level, cycling has a much higher health benefit that it poses an injury risk. personally, and as a country, the benefits outweigh the risks between 10 and 100 to 1
plus there are, at all levels, things that you/we can do that will have a much bigger impact on cyclist safety. effectively encouraging more people to ride bikes and building better infrastructure will keep cyclists much safer than mandating helmet use. at an individual level, good route choice (i.e. ride on a bike lane or quiet street vs. riding on an arterial) will have an exponentially higher impact on your personal safety than wearing a helmet does
finally, consider: motorists and pedestrians receive the same, if not greater, benefits to head-safety from wearing helmets as do cyclists. knowing that, are you going to put on a helmet when you go for a walk, or get behind the wheel of a car?
"Nobody really gives a shit that you don't like the thing that you have no firsthand experience with." Dave