My physic may indeed be off, but that isn't really the point as you acknowledged. Rather it is the potential for death.
Now, my personnel opinion aside, this is just an example of how the courts (state) would view this issue.
Trees are part of nature and an expected hazard. Telephone poles are an expected hazard as well. A concrete mailbox is not. For example, if you go to the ball park and are hit by a baseball that went wild after being hit by the batter, you have no grounds to sue anyone. The risk of getting hit by a baseball at a ball game is an expected hazard. Now, if one of the players intentionally threw the baseball in the stands and you are hit with it, well you have grounds to sue the thrower. It can not be reasonably expected to have a baseball intentionally thrown at you during a ball game if you are a spectator in the stands.
Likewise, if you are driving on the road, you can expect trees, and telephone poles, and even mailboxes. However, a reinforced concrete mailbox is built with the intent of damaging vehicles that may run into it and would not be reasonably expected.
One last point, although a clean conscience is a noble goal, it is not the intent of the law. You may have clean conscience setting up a hazard, assuming if the individual doesn't break the law, the hazard won't hurt anyone. However, the law is not concerned with your conscience, but rather the potential impact of your actions and whether you could have reasonable foreseen them.
In the cases describe, I think it is reasonable to assume the intent of the builder of the reinforced mailbox was to damage a vehicle and a rational person could foresee the potential danger of the reinforced mailbox.
Yeah, but the law is wrong about some of this. For example, putting man-traps inside your home isn't allowed--as if the burglar is entitled to expect decent working conditions.
If property rights were properly respected, it would be allowed to put land-mines on your property, assuming only that proper precautions were taken to prevent someone, say, entering your property by accident. In a fenced and posted property, a minefield makes perfect sense.
Would your statements apply to people putting up posts across the front of their yard, or small boulders, or a very sturdy fence - could they also be culpable if someone was hurt?
I have been considering constructing a ferrocrete mailbox structure simply for the low maintenance aspects. I have no disire to harm, injure, or even ding a car. I have no desire for anything to ever hit it, I just do not want to have to rebuild it again like I have had to replace both metal and wood posts for my mailbox over the years due to rust and rot. (And the one time the neighbor kid took a crutch and mashed my metal mailbox rather effectively, but his mom bought us a new one.)
Is it more a matter of disguise, or simply a matter of immoveability? Is there a minimum distance that massive lawn ornaments have to be clear of the road, or do lawn-redecorating drivers have the legally allowable expectation to be able to nearly sideswipe the front porch without hitting anything in the yard that would be a danger to them, possibly besides obvious large trees?