Most are designed to snap off when hit. There is actually a real LOT that goes into roadway sign engineering and we have learned that it is the safest option. It's called a breakaway feature.
http://safety.fhwa.dot.gov/roadway_dept/policy_guide/road_ha...
>Roadside signs, utility poles, lighting structures, traffic signals, railroad warning devices, motorist-aid callboxes, mailboxes, and other rigid objects can become deadly roadside hazards if placed where run-off-the-road vehicles can strike them. While it is preferable to maintain obstacle-free roadside clear zones, this is not always a practical option. When rigid objects cannot be removed or relocated, potential crash impacts can be mitigated by specifying breakaway features, or by shielding the object with a longitudinal barrier or crash cushion.
And here's a video of a light pole breakaway feature being tested. :)
http://www.youtube.com/watch?v=bbg5NkbuN6I
