Some states require a specific amount of retroreflective material on a helmet. Your local motor-vehicle department can give you exact information on the location and number of square inches of retroreflective material required in your state.
Wearing a helmet properly strapped on your head is mandatory in many states. Laws are always changing, so double-check with the state department of motor vehicles for the most current information. Are you planning a tour through several states? Plan to wear your helmet in all states, regardless, and remember that laws apply to travelers as well as residents. Don't leave home without the information you need.
Are you aware that regulations for motorcyclists vary from state to state?
Go here for a very informative article posted on the NHTSA website providing detailed information about the test used.
Go here for Articles and Videos that provide additional information about the safety of DOT-certified helmets.