Giving drivers sight of pedestrians is good, but giving drivers to much open sight lines is bad. Open sight lines are exactly what make people drive faster. Mailbox near the road seem like a great idea to me. It communicates that people might be here, creates a visual obstacle that will get people to drive slower, and if you hit one, it doesn't cause a shit ton of damage (unlike, say, a tree).
As a counterpoint, having mailboxes right next to the road is a great way to wind up with your mailbox absolutely obliterated by the snowplow as it goes by...
