I knew someone involved in a sizable study looking at what factors had the biggest effects over time. There were plenty of small factors around certain safety requirements/developments or traffic control, but there were only two big factors: mandatory seat belts and extremely graphic advertisments, aka the TAC ads.
The TAC ads basically showed you what accidents look like in real life, and what you look like after an accident. None of this soft touch crap - the state next door ran an anti-speeding campaign where they had people extending their little finger at speeding drivers (as in 'little dick syndrome'). Absolutely laughable and the folks who are the bulk of the problem - young men - ignore crap like that.
But when you see an advert that has someone's head going through a windshield in slo-mo, or a motorcycle rider coming off his bike crotch-first into an oncoming van, or a hospital scene with a bloke in traction, it helps dissolve the 'it won't happen to me' mindset. Not altogether, but it's no longer conveniently put out of mind.
I'm at work so I can't find some youtubes to demonstrate, but the ads are usually surprisingly 'real'. Needless to say, the ads are usually quite shocking to people exposed to them for the first time.