Probably less than you think:
People who are forced by legislation to wear a bicycle helmet … may not wear the helmet correctly, seeking only to comply with the law and avoid a fine. Secondly, their behaviour may change as a consequence of wearing a helmet through “risk compensation,” a phenomenon [where increasing safety measures will lead people to engage in more risky behaviors]. One study — albeit with a single author and subject—suggests that drivers give larger clearance to cyclists without a helmet.
Vaughan Bell notes how, in general, safety measures may be offset by the behavioral changes they inspire:
Known as self-licensing [this effect] is where people will allow themselves to indulge in more harmful behaviour after doing something ‘good’. For example, people who take health supplements are more likely to engage in unhealthy behaviours as a result.