Would you wear a helmet if you were riding a motorbike? You may be a great cyclist, but there are some shocking drivers out there.
The British Dental Association is quoted by the pro-compulsion "Headway" charity as evidence for their stance.
The BDA advocates full face helmets for protection of the face, as the present designs are woefully inadequate in facial protection.
Secondly the functional design and testing of helmet only "proves" their worth up to impact speeds of 12 mph. In an impact with a car may (or may not) be able to prevent a head injury.
For vehicle type collisions we really need helmets more akin to motor cycle helmets in order to gain adequate protection from head injury at these types of impact.