Mountain and road biking are very different. Unless you riding roads in exceptionally isolated areas, there'a always traffic. So unless you fly off the road and down into a ravine, odds are that any crash will leave you along the road, where passers by will render aid, or at least phone the police.

Mtn biking, like solo hiking has the risk of injury, where nobody may pass for hours, days, or even weeks. I'm very comfortable alone on roads anywhere, but hate to mtn bike solo, even in fairly popular parks.