According to AAA, converting traditional intersections to roundabouts covers 30 percent of the fatality and injury reductions. They do this by drastically reducing the number of ways and locations cars can collide with other cars or pedestrians. What points of collision remain for cars are also less severe, since T-bone or head-on collisions are unlikely.
The other areas of improvement AAA lists are fairly commonsense. Adding roadside barriers and removing roadside objects comprise another 20 percent of safety gains, as do adding sidewalks and pedestrian crosswalks with signals. The remaining safety improvements would come from additional median barriers on highways, shoulder and centerline rumble strips, and wider highway shoulders.
AAA estimates the cost of these infrastructure changes at $146 billion. That includes an upfront cost of $134 billion, and the rest of the money would be used for maintaining the upgrades over the following years. If you're interested in digging deep into the AAA report, you can view it here.