RE: Metrics, Delaware has its own metric highway.
Details on metric signs on US highways can be found here.
Regarding checkpoints, my mother was a naturalized US citizen. She was once given a hard time by CBP because she didn't carry her naturalization papers with her some 25 years after she became a US citizen. This was on a mid-80s entry to the US from Canada. They seemed to give her a hard time because they could, not because they had any doubt that she was a citizen.