Until there is a distinct change in driver attitudes, riding without a helmet in most of the US is just nuts. Sure, there are places like Portland with dedicated bike lanes and an overall emphasis on pro-bike culture, but it is a rare spot.

In the bay area, there is a very large biking community -- road, mountain, around town, etc. The problem is, the infrastructure isn't there for bikes compared to Amsterdam. We have "bike lanes" that any vehicle can enter within, say, about 150' to make a turn onto another street. Vehicles don't look for bikes. The bike ways in Amsterdam, certain parts of Munich, and other towns are often separate from the roads. When not, people expect them and are courteous to them much more than here.