The US is a huge country. Just to replace the road related signs alone means replacing or modifying hundreds of thousands of signs. There's mile markers (at least 48,000 alone given the 46,876 miles of interstates in the US), exit signs, distance to destination signs, and speed limit signs. These signs are rarely replaced unless they're damaged so 8 years would still require a lot of unexpected maintenance.

Why? Most industries that should be using metric already do in the US. Cars use metric fasteners, etc. When i worked in a lab, everything was metric.

Imperial is only really used in consumer facing situations. If we forced everything over to metric, would it make that much of a difference? Would the US economy suddenly surge due to a more efficient system? Unlikely.

I think the US could gradually move to metric, but as with the UK, there's no need to replace miles as a distance measurement. In some ways, I think it'd be nice to always keep mph/miles... but otherwise, for weights and measures, metric would be superior.

And hey, if it's good enough for NASA, it's good enough for the rest of the states. ;)

Worst hit, would probable be the automobile industry. But I'd say there'd be advantages to metrification there too.