I'm not sure there's a whole lot that can be done, legally. Our road is eight feet wide with a ditch on one side and a stone wall on the other, the whole thing climbs at 22%, and tourists from the neighboring ski/tourist trap walk down it all the time. At least a couple times each year we have a near miss where some speeding punk takes the road at 50mph+ and nearly takes out a pedestrian or mtn biker...yet the town won't bother posting the road.
So every once in a while I have an accident with the rear blade on the tractor -- actually a whole string of little accidents -- and the road turns into washboard. Tends to slow most down, but some of the larger SUVs don't appear to even notice the road is rough.
One summer I put a huge poplar log next to my driveway. The thing was about eight feet long and two feet in diameter but only weighed about thirty pounds. If someone sped up the road I'd lift that log (straining like it weighed 300#) and drop it across the road as the punk came back down the hill. /forums/images/graemlins/grin.gif Then I'd growl like the Incredible Hulk while lecturing them on how their lives would change if they ran over a family here on vacation. Most were so in awe of the log trick they got really meek and polite.
But mine's a deadend road, and it sounds like yours is a shortcut. In my experience, shortcuts are raceways.
Pete