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I note that in the USA the "default" urban speed limit seems to be fairly universally 25 mph, not 30. That's just an impression, not based on any perusal of all the states' laws. I don't know whether the "Uniform Vehicle Code", a sort of moral example to encourage sensible state laws, has any numerical value. I suspect not. The 25 mph value has been around for a long time. It was long established when I first went to the USA forty-four years ago. All the fifty states must have had their own history of how their laws evolved. That 25 mph is generally set by state law, but, just as with the British 30 mph, a street might be given a different limit. It seems to be the standard practice of American traffic engineers to choose a speed limit by observing the speeds that motorists actually follow, and then setting the limit to turn one driver in seven into a criminal. There's some handwaving about standard deviations involved with that, to make it sound scientific, but it does strike me as being an odd way to go about things Jeremy Parker