too much typing—since 2003
why the metric system never caught on in the US
When I was a kid, I remember a big push (from my math teachers in particular) to get everyone to become familiar with the metric system. I remember baseball stadiums (even those not in Canada) suddenly sprouted metric measurements on their outfield fences. Unfortunately, when those signs said things like "106.68 m," they reinforced the notion that the metric system required a nerdy precision, what with carrying things out to a couple-few decimal places. This was completely unnecessary, since in most cases we do not need an exact measurement. I doubt that your typical outfield fence that says "350 ft." is exactly 350 feet - it may be 350 feet give or take a few inches, and given that the fences are rarely curved in such a manner as to be equidistant from home plate, at one point in the "350 ft." sign they will be 349 feet and 6 inches and at the other end of the sign maybe 350 feet and 4 inches.
What should have been emphasized wasn't conversion but familiarity. We really don't need to know most measurements to the centimeter, and it would have been far more effective to know that a meter is a little longer than a yard, a kilometer's a little less than two-thirds of a mile, etc. - but beyond that, we become familiar with these units in reference to what they measure. We're all used to 2-liter beverage containers, for example (for some reason, that measurement took hold). And if we got used to how fast 80 kph felt, or how long it took to drive 125 km on the freeway, who cares about conversion?