Here in Australia, wearing an Australian Standards-approved helmet is mandatory when riding a bicycle. I know a few people who won’t wear bicycle helmets because of the bulk of the helmet, or because it might muss up their hair-do :) So it’s either risk a fine if you’re caught (as well as risking head injuries), or don’t ride a bicycle.
The Hovding Invisible Bicycle Helmet is an airbag contained within a fabric collar that deploys in the event of an accident. The colour and fabric of the collar can be changed to match the weather or your fashion sense. The collar includes a “black box recorder” which allows you to analyse crash data.
If you want to view a helmet deployment, check it out here.
It’s not approved in Australia, but it’s available for 3998 SEK ($575) in Sweden.