Have "helmet" will travel

Motorcycles are ubiquitous here. And by ubiquitous I mean really, REALLY annoying. There appear to be no rules governing motorcycle traffic patterns.

There must be rules requiring helmets, however, because virtually all motorcyclists and riders are seen wearing them. The type and use of the helmet seems not to be very regulated because I have seen all of the following serving as motorcycle headgear:

The standard motorcycle helmet. Many of these are seen to be in less than stellar condition and even more frequent is to see riders with the helmet tipped back on their head so they are not looking through visor but rather under the chin of the helmet.

The standard bicycle helmet. This is especially common on motorcycle passengers which I just can't figure. First, passengers tend to be children or girlfriends/wives. Wouldn't you want to give them the superior protection? Furthermore, the driver has something to hold on to and is more likely to know when the accident is going to happen. Give him the chintzy helmet! (By the way, a common passenger arrangement is smallest child in front of driver, medium child behind driver, and wife behind that.)

I have also witnessed the following as helmets:

The Batting Helmet

The Hard Hat

And for the first time today...

The Equestrian Riding Helmet