I know this is just a pipe dream, but for those of us that live in the Commonwealth (defender of the faith etc etc etc etc..)
In each of these countries the post office is federally controlled--hence a Crown Corporation (Royal Mail in the UK, Canada Post (crown corp) etc).
I believe there is some statute (or common law precedent) whereby it states that "nothing shall impede the Queen's mail."
So....lets say you have a sealed letter in your car. Don't those pesky stop signs, traffic signals, speed limits impede the delivery of Her Majesty's mail? Of course you would natually have to testify in Court that you were driving to the post office (and nothing was going to impede the delivery of that letter--don't want to pi$$ of the Queen you know..)