what should the speed limit be?

the lower the safer, right?

nope.

research shows that the safest speed limit is the speed at or below which 85% of drivers will go on that road. artificially lowering the speed limit below that speed does not slow drivers down, and increases accidents because some people will artificially slow down, causing a poor flow of traffic and increasing the chance of collision.

For more details... http://www.motorists.org/issues/speed/speedfaq.html