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1) 1 car for every ten miles = less than 1 second of following distance
2) most people can not accuratlly judge a car length distance of more than one or two at best

This is why the new rules were developed. If your peripheral vision is so poor that you can spot a marker to the side of your car as you pass, you dont belong on the road. its a rough estimate, you dont have to hit your mark within in a tenth of a second.