Do helmets make riding a bike safer? The evidence is mixed. In a crash, they certainly help. But they may cause more accidents, or prevent people from riding in the first place. Retrospective analyses of helmet use and mandatory helmet laws have found inconclusive overall effects.
But as Aaron Carroll explains, there was a recent study on a different bike safety measure: permanent (that is, always on) running lights. This study was a randomized-controlled trial — a much more powerful study design than the case-control studies on helmet use. The result was astonishing. The incidence of overall accidents for those using the lights was 19 percent lower — and 47 percent lower for multi-party accidents where the cyclist was injured.
That single study is far better evidence than anything that exists for helmets. Bike enthusiasts, take note. --Ryan Cooper