Han Maenen wrote: "I do know, that in the USA there are 25 mph speed limit zones; I saw that often on TV."
In California, 25 mph is used as the speed limit in residential areas and in school zones (when the schoolchildren are actually present). Other limits are 35, 40, 45, 50, 55, 60, 65 and 70. For state highways, the choice between 60 and 65 varies by county. Most of the highways are 65 mph (including many non-freeway stretches with only one lane in either direction). Interstate 5 and one or two connecting freeways are 70 mph. The de facto speed limit in 65 mph and 70 mph zones tends to be around 80 mph (which the California Highway Patrol tolerates). Interestingly 80 mph is close to the German "Richtgeschwindigkeit" (appropriate speed) of 130 km/h for the Autobahns. (Almost nobody actually drives that slowly except in those sections that have posted limits.) Given the size of the mile, I think the speed limits ending in 5 can be justified. I agree with Han, though, that there's not too much justification for anything other than multiples of 10 for km/h (other than for the very low limits in places like shopping center parking lots). Interestingly, there is a winding road in Saratoga, CA, which has a very curvy section posted with a hard-to-ignore speed limit of 21 mph in one direction and 22 mph in the other direction. The purpose is not to set the limit exactly at those levels, but to draw motorists' attention, through a little humor, to the need to drive carefully. Very clever, in my opinion. It certainly got my attention the first time I drove along that road. Bill Potts, CMS Roseville, CA http://metric1.org [SI Navigator]
