There hasn't been much research in the field of hitchhiking. Bernd 'The Thumb' Wechner wrote an article about this a few years ago and you can find it here on digihitch:
If you want to find the odds of injury or death for a hitchhiker, just look at the odds of injury or death for a person who drives over 20,000 miles per year. They're about the same, since the biggest risk has always been traffic accidents and drunks.
I've browsed the 1974 study Bernd mentions, done by California Highway Patrol. Officers interviewed every hitchhiker they could find in 1973 and found...
hitchhiking was responsible for no more crimes or violence than any other mode of transportation. The most common injury involved bad driving.
And keep in mind this was the HEYDAY of hitchhiking. 1973 in California had tens of thousands of hitchers on the highways.
For those who say 'times have changed, it's more dangerous out there', don't be fooled. They said the same thing in the 1920s, when automobile clubs and business owners were trying to scare drivers into believing all hitchhikers were homicidal maniacs or disrespecting bums. And they said it again in the 50s, only then they were 'reefer addicts and immoral rebels'. They said it again in the 70s-- that they were dirty hippies and druggies.
Times aren't so different. There have always been drugs and freeloaders. If you walk out your front door you'll find them. But not all hitchhikers -or drivers- use drugs or freeload.
Sure it can be dangerous on the open road. Because you're dealing with fast-moving cars and sometimes a person who's just plain stupid or unsafe.
... but the day that there is substantiated proof that hitchhiking is no more or less safe than riding a bicycle is the day hitchhiking has become institutionalized and drab, with ID cards and government regulation. As for me... I prefer a little risk.