I've had a helmet for a good few years but only started using it daily since I electrified my bike. The higher speeds I'm doing make it a sensible thing to do. I cycled for years without one though. If you're doing 15mph or so on tarmac the risks are relatively low depending on traffic conditions. As the risk increases either through speed, traffic or terrain (mountain biking for example) then there comes a point where it's prudent to wear extra protection.
Incidentally I have, personally, noticed no difference at all in the behaviour of drivers around me with/without a helmet.
Incidentally I have, personally, noticed no difference at all in the behaviour of drivers around me with/without a helmet.