Celtic: Can you wear derbies or caps for the actual event though?
The rule applies to the event/test/class itself. In other words, national (USEF) dressage riders have to wear helmets during dressage tests starting Mar 1.
These are USEF (national) rules. Rather than being an entirely new rule, it is just that helmets are now required for more types of USEF events/classes/tests.
There are a handful of riding or driving competitions in the US run under international (FEI) rules. Their helmet rules are not quite the same as the national federation.
Some dressage shows offer both national (USEF) and international (FEI) classes. If the rider is entered in both national and international classes, I THINK s/he has to follow the national (USEF) rules for ALL the classes he rides in.
There are a great many wonderful traditions in riding and driving. Helmetless just isn't one of them.
I think many people hope that the competition rules will get people in the habit of wearing helmets at home too.
Years ago, we never wore helmets, except for maybe those pretty useless hunt caps with little padding, a flexible metal shell and a flimsy elastic strap that went under your jaw. I don't think they were as effective as the helmets we have now.
For about 20 years I had a 'Caliente' - the kind of helmet eventers wore for many years. I think that was a lot better than the old hunt caps, but the newer ones are lighter and have more coverage in one key area - low at the back of the head.
For some reason that old Caliente seemed to be a haven for every critter and vermin in the barn. I was positively hygenic about it for a while and had proper hysterics about it, but after a time I was like all the other old leathernecks there - toss out the mouse poop and strap on the helmet.