I commute by bike on roads shared with cars every day and wear a helmet. When I ride in the whole way to work, I tend to average about 15mph, which is probably about 18mph if I count only time actually moving. Some cyclists can ride far faster than this. Most cyclists ride slower than this. There are far more occasional riders or people taking a mountain bike to Caltrain than there are lycra-suited enthusiasts.

I have never seen any reference to bike helmets being "rated" for a specific speed or type of impact. Bike helmets are approved based on a single test carried out by the CSPC which is not calibrated to correspond to any particular speed or type of impact. I don't consider the method of testing helmets particularly important to the well-supported fact that these helmets significantly reduce the risk of serious head injury and death.