I've run and biked a lot over the years. This has meant getting my fair share of close calls with vehicles and occasional falls. I've been fortunate to not have had any serious incidents, and for this I am grateful.
I spent this past Saturday going to a race out of my area. While there, I met an acquaintance (from the web) for the first time. He was reminiscing with an old running friend about a mutual friend who'd been hit by a car and killed. Unfortunately, this is not a terribly unusual story. While I wasn't close to anyone killed by a car, I know many cyclists and runners who have lost close friends to these kinds of incidents.
When I bike I wear a helmet. It certainly doesn't make you invincible, but it goes a long way to improving the odds of surviving a serious crash. I've seen some of the helmets of people who've cracked them in two, and you know a lot of energy was absorbed by that helmet as it broke, and that energy was spread around the skull, and the skull was not fractured. It also prevented the brain inside the skull from being pulled or twisted as severely, thus reducing the likelihood of a serious brain injury. The standard comment when you see this is, "Imagine if your skull had been hit directly." Those of us who have been through this conversation know what the consequences would have been.
A few months back, I was in a Starbucks, and made a bit of conversation with another customer. The upshot was that he'd just returned from visiting his father in a hospital out of state. His father was doing some work on or near the roof, and his ladder tipped. He'd suffered a serious brain injury and it was unclear how fully he'd recover. At the time, he was still hallucinating some of the time and, as a result, had to be restrained for both his safety and that of others.
I'm a big proponent of helmet use on bikes (and motorcycles). I think we should all make reasonable choices to limit risks we take, but none of us can (or should) lead a risk free life. It's both impossible, but it can also prevent us from truly living. So, try to manage risk - as the cyclists say, "It's not, 'if I fall', it's 'when I fall'." You can't avoid all the accidents, but you can limit the damage when they happen.
Whatever you do, be safe!