I live at the top of a mountain and the fast way down is a two-mile, steep, straight, divided, wide and well-paved road. Unless I apply brakes, and I generally don't unless it is wet, and it's usually not wet, I am going 45-55 mph without thinking about it. I normally ride at about 85 lbs front/rear. If I for some reason have my tires inflated to race pressure, say 120/130 front/rear, and drop into a tuck, I could easily hit 60. Faster if I pushed it. This is normal for a road race descent; although rarely for extended times.
I have crashed at speed, in a race, on a very fast descent. Although my helmet didn't have a scratch, I do remember "coming to" as I was being dragged by the armpits - broken collarbone and all - off the road by a teammate. I think in the moment, once I was over my handlebars and had my arms extended in the Superman position, my mind said 'fxxx it, let's not remember this' and checked out. Honestly the last thing I remember is my front wheel hitting the back of the guy who went down in front of me, putting a few of his ribs into his lungs. We had a morbid laugh in the ambulance on the way to the hospital. Well I laughed; he kind of wheezed and choked on bloody spit.
Anyway (aside from racing where you have to go as fast as possible regardless of the hazards or wtf are you there for?) if there is no one in front of me, I tend to consider the descending risk in terms of road rash potential (how smooth is the pavement?) and roadside hazards I might strike (parked cars, fence posts, newspaper racks, rocks and the impact they will make on my body) if I were to find myself out-of-control at some speed. And I adjust my speed accordingly. A lot also depends on the condition of my front tire (especially now that I ride clinchers), a blowout of which really decreases the maneuverability of a bike at speed.
But to be honest, I have never considered the impact of higher speeds on the effectiveness of a helmet. I know it is way too simplistic an analysis, but the fact is when you fall you only fall a few feet; it's the sliiiiiiiiiiiiiiiding that does the damage.
I probably have a different mindset because I have raced since the early seventies; hardshell helmets weren't required until 1986 in the USA and I don't think pros had to wear helmets at all until a few years later. And head injuries in racing were not common back then. Probably because bike racing was not very popular; only really good cyclists were interested, and they were probably really good because they were naturals. And naturals are less likely to get hurt. Just my opinion. I'm not bragging; I'm more likely to hurt myself in the kitchen than I am on a bicycle going any speed. Nowadays, however, cycling is the new golf. Everybody has an expensive road bike and lycra shorts! And a lot of the guys I ride with may be strong, but they really can't ride a bike safely and have no concept of safe, defensive, hazard-avoidance riding. I have tried to transmit my experience and insight to new guys but it is very difficult to do so. And this pool of cyclists is more likely to include guys who aren't "naturals".
I have also noticed that guys who hold the bars tight tend to crash more, as they put the center of gravity forward to the front wheel with that death-grip. Guys who only rest their hands on the bars (like yours truly) keep the cog back on the rear wheel, crash less and are more likely to get their arms out in front when they do crash. This sacrifices the circuit-breakers (road rash and the odd collarbone) but protects from separated shoulders and head injuries.
Are you a handlebar-grabber? Go to a grassy park, practicing riding with your hands off the bars until you can ride in smaller and smaller circles without touching the bars or leaning forward. You are learning to steer with your hips.
I just thought I'd share some insight but, to be honest, now that you bring it up I will be thinking more and more about speed vs crash ratings for headgear.
. I am more scared on a road bike at those speeds than I am on my mtb. JMHO & YMMV.
