Bike speeds > 20km/h are normal and easily achievable for the untrained on flat ground. Downhill, you'll quickly go past 30km/h. This isn't something that you're doing while running for longer periods of time while you're still half-asleep.

Helmets annoy me too, but "the fastest runners can peak at speeds higher than what I reach every other day on my bike and they don't wear helmets" is a weird argument.

For rides where I go faster, I will take my road bike and wear a helmet.

For rides where I go slower, I would prefer not to wear a helmet, but mandatory helmet laws don’t make that distinction.

As you alluded, even between bike trips, I weigh the risks differently when I think about a casual spin down a protected lane or a recreational trail as compared to an aggressive commute inches apart from vehicle traffic.

My local bike share program has started blurring things even more for me lately as they introduce progressively zippier e-bikes. Any more, it’s trivial for me to move with traffic on these huge heavy clunkers of bikes, while to my impulses it still feels like the low-effort casual kind of ride I’d instinctively rate as not warranting a helmet.

Still, I’m glad to be judging for myself how to mitigate the specific risks of a specific ride.