The shorts and sandals option was kind of a joke, I'm glad no one picked it
.
I was walking on the road the other day and had traffic passing me at 25-30mph, really hit home how fast I'm going on the bike. You can get used to any speed if your platform is stable, but kinectic energy (and thus impact energy if you have an accident) increases by the square of velocity.
Example: If my bike and I weigh 250lbs, this is our kinetic energy at various speeds:
15mph - 2549 Joules
20mph - 4532 Joules
25mph - 7,082 Joules
30mph - 10,198 Joules
As you can see, twice the speed, 4 times as much energy! Standard off the rack helmets just aren't made for these speeds. Even Virginia Tech's helmet testing method has a maximum speed of 16mph.
(Link Removed - No Longer Exists)