Highway 21 9mm Helmet If you have to cover your dome, and let's face it, you should, then the 9MM by Highway 21 is a great way to do just that. Classic helmet design combined with premium material for a helmet that will protect your head, neck, and keep Johnny Law at bay. Shell: Durable, lightweight ABS shell Expanded polystyrene interior Interior: Diamond-stitched Coolmax liner Safety: DOT approved Dual D-ring chin strap Other Features: Includes protective helmet bag Note: Customers have found this helmet fits big and recommend sizing down if in between sizes. This helmet also has a round head shape, which means there is less room from front to back and more room on the sides of the head.