• Head injury is a leading cause of death in motorcycle accidents, according to the NHTSA.Wilfried Krecichwost/Digital Vision/Getty Images
  
  Wearing a DOT-compliant helmet is the best way to protect your head and to help prevent serious injury in case of a crash on a motorcycle or bicycle. The Department of Transportation (DOT) regulates the manufacturing, the proper fitting and the laws regarding wearing a helmet when riding a motorcycle or bicycle.
  
  

DOT-Compliant Helmets

• The National Highway Traffic Safety Administration (NHTSA), a division of the DOT, establishes standards for motorcycle and bicycle helmets to make sure riders are protected to a certain degree when involved in a crash. A DOT-compliant motorcycle helmet is at least 1 inch thick and covers the rider's ears. Bicycle helmets have similar standards --- they must be 1 inch and cover the forehead --- that the U.S. Consumer Product Safety Commission (CPSC) issues. Any DOT-compliant helmet should be marked with an identifying DOT sticker.
  
  

Helmet Fitting

• Owning a DOT-compliant helmet is not enough to ensure safety. Making sure that helmet fits properly is also key. The NHTSA has certain guidelines to follow when fitting a helmet. The helmet should be level on the head, covering the forehead and not tilting. The straps and buckles should be secure, and the side straps should form a "V" under the ears. When on, the helmet should fit comfortably. If the helmet moves around; if there's a distance of more than two fingers between the eyebrows and the helmet lip; or if the helmet hangs into the eyes, remove it and readjust for a better fit.
  
  

Helmet-Wearing Laws

• Though the Department of Transportation recommends that everyone riding a motorcycle or bicycle wear a helmet, helmet-wearing laws vary by state. As of June 2009, there are 21 states that require motorcyclists to wear helmets at all times when riding. Some states require riders only under the age of 18 to wear a helmet; others have no requirements at all. Bicycle laws are similar in that the helmet laws vary: Most states require minors or children to wear helmets, some states require all users to wear helmets, and some states have no requirements.