[longtermmemmory]

FMVSS (federal motor vehicle safety standard) 218 only rates helmets to a a speed of 13mph. There are no neck standards.

[Smokin' Joe]

That's because hitting a fixed object at 55 MPH (or more), and even slower speeds, will likely result in spinal injuries or closed head trauma which will be fatal anyway.

For the speeds involved, bicycle helmets make more sense (protection from low-speed impact trauma).