I wonder if you could design subtle traffic calming like long wavy roads that you only notice at speed. That is, when you travel at the speed limit, you don't notice anything unusual about the road, but as soon as you go over the limit, it starts feeling funny because you're encountering subtle bumps and curves just a bit too quickly. It should make you subconsciously want to slow down and obey the limit.
Mokalus of Borg
PS - In practice, it would probably be too small a change to notice and be effective.
PPS - But it would be interesting to try.