and helmets aren't made for impacts with motor vehicles.

Even in a study that showed decreased injury from wearing helmets the author said, helmets are not designed to protect the rider in a fall which involves other vehicles.

"when a cyclist is knocked off by another vehicle, this frequently results in the head being spun and subjected to torsional effects. One consequence of this is that they tend not to hit the ground as cleanly as children who are typically involved in low-impact, non-twisting injuries," said Dr Sheikh."

http://www.imperial.ac.uk/college.asp?P=2250