I have a MICS (Medical Information Carrier Services) ID on my motorcycle helmet. Same idea as Road ID, either a local piece of information or an Internet based one. The one I have is a plastic sleeve that sticks to your helmet with the ID info inside. It used to be free for the asking, but that got abused. It is still free with any order, or $1.99 each with free shipping from
Cycle Gadgets. Next time I order from them I plan on getting one for all of our bicycle helmets.
MICS has their own bicycle version, too, that is a red sticker for the outside of the helmet telling you to look inside the helmet for the medical information. This version is linked from the above web page.