Surprisingly, the U.S. does use the metric system a great deal more than might be supposed. As mentioned in the question, scientists and medical doctors use the metric system, but in general, the "common people" do not. And, why not? I believe that it is because they think that they will have to convert every measure they are using now, by themselves, into metric which will involve (at the least) division and multiplication by decimals, or (and here is the real fear) division and multiplication by FRACTIONS. Our country isn't have a hate affair with the metric system, they experience revulsion at the very thought of having to use fractions again.
Some problems that occur are when two necessary measures are not both in the same system. For example, we talk about a distance being 150 kilometers, but the speed signs on the roadways are in miles. This is despairingly frustrating when trying to figure out when one is going to arrive.
One problem that the United States experiences in the world is that trade in the world is a fast moving exchange between countries. Citizens involved in trading with the United States where weight or size is concerned, roll their eyes (I've seen it), sigh and grab their conversion charts (or calculators or computers). Our foreign trade suffers from individual errors in conversion because we don't use the same system that everyone else in the world uses. Don't forget that we have to convert their measures to the standard system.
To resolve the gap between the U.S. measurement system and that of the rest of the world is that we just need to bite the bullet and continue installing metric units wherever and whenever we can. Retraining our citizens to use the metric system needs to first happen in the schools. If the teachers aren't teaching metric, then the students aren't learning metric. Secondly, our governmental departments responsible for standardized measures, should continue listing things in both measures (my soda can lists the contents as 12 fl. oz. and 355 ml). Slow and steady wins the measurement race.
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