That would be the last place to start, as it would cost a fortune to replace all of the highway signs. Not only that, but also all of the mile markers, for which most states have every 1/10 of a mile. Moreover, contrary to what some people have implied, the numbers are generally not painted on, they're fabricated from other materials and overlaid. And for what? So we can convert the length of our commute into a multiple of our height, or something else of the sort? Yes, it's absolutely absurd that there are 12 inches in a foot, 3 feet in a yard, and I-don't-even-know-how-many yards in a mile (and yes, I've heard of Google/Wikipedia; but I just don't care). The truth is, I never need to convert inches into miles. You measure human-scale things in feet and inches, travelling distances in miles.
On the other hand, you know where we should start: volumetric measurements. I have frequently had a recipe that takes some number of teaspoons of a liquid, while having measuring cups measured in (naturally) cups, and nutritional information in ounces. Oh, and keep in mind that most tea spoons are significantly larger than a teaspoon. And then there's tablespoons, pints, quarts, gallons, barrels, and who knows what else. This is a lot harder to keep straight, and unlike miles to inches, sometimes you actually need to convert between these.
Add into the mix the problem that pints differ from place to place (either 16 or 20 oz), and "ounce" is both a volumetric measure and a weight measure. Obviously, if you have something that's clearly a solid or a liquid, it's clear which is which. But what about, say, frozen yogurt. When the self-serve froyo place sells by the ounce, and posts calories by the ounce, it would only be reasonable to think that these are the same ounces. It would also be wrong.
Moreover, in the case of volumetric measures, not only do you have a real problem, but an easier solution: most of the containers that hold liquids are disposable anyways, and constantly manufactured (i.e. food). All that would need to be done is to make containers that are metric-sized, and printed with metric labels, rather than Imperial. In fact, we're closer to that already. By law, all wine and distilled alcohol must be sold in one of several metric sizes (for distilled, it is 375 mL, 750 mL, 1L, 1.75 L, if I recall correctly). Soda is frequently sold in 2 L bottles.
Do that, let people see that metric actually saves time and hassle, and then go about changing other measurements. Weight would probably be the easiest to transition next, followed by lengths for things other than highway signs. (No one will care that they can't easily convert meters into miles, just as they don't care that they can't convert feet into miles). But please don't try to start with highway signs. Or bother with highway signs at all, for that matter. They are the death of metricfication in the US, and insistence on them is only counterproductive to the rest of your goals.