Speed limit has no effect on the average speed of traffic, as the United States Department of Transportation found out when it did a study on the subject. Varying the speed limit as much as twenty mph in either direction yielded no change in speed. Instead, it was later found drivers unconsciously decide how fast to go based on the road geometry/surrounding factors versus the perceived risk of going faster. So, if you want to slow people down, you make the road appear more dangerous and harder to navigate. One large town in Europe did this by removing all traffic lights and related signery, replacing them with unregulated traffic circles. The end result is that people are not only driving slower, but traffic flows smoothly and accidents have nearly vanished. Amazing what people do when they think there is more danger.

Thus, it's pointless to have a speedometer doing such things except for pure novelty - speed limits are an arbitrary number that doesn't affect anything.

Thanks for your comment toastydeath! I think your understood my idea a little bit wrong way. I'm not limiting speed or pulling speed limits down. My point is to visualise better way to driver what he can achieve driving faster than it is necessary. Some luxury cars has this display on their driving computers, it shows how many minutes you have to travel.

You are true that those traffic circles are working. In Finland we have had trend to change most of crossroads to traffic circles during last years.