Also re autobahn.

Having a speed limit at all creates a sample of ppl that have varying degrees of adherence to it. For example...you'll have ppl that completely ignore it and go faster/slower, and ppl that stick to it like glue. This causes a natural difference in speed between ppl and can be dangerous. The autobahn eliminates this mandate...and so most ppl drive at the speed they feel is comfortable and safe...which averages out pretty well.

Also this Speed Limits: Frequently Asked Questions is pretty informative