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TPMS has been law in the US since 2007. People might be driving around with under inflated tires, but there is a light on if they are.

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My cold tire pressure is supposed to be 33PSI, my TPMS doesn't alert until it's below 28PSI, so there is a significant delta still.

The TREAD law does not specify when TPMS must alert, other than the tire being 'significantly underinflated'.

The NHTSA has determined that the requirement is that TPMS alerts for 25% underinflation across any combination of tires, or 30% underinflation in a single tire.[1] Originally they wanted 20-25%, but determined that cheap TPMS sensors were not good enough to meet that threshold and did not want to impose the cost of better sensors.

[1]: https://www.nhtsa.gov/sites/nhtsa.gov/files/fmvss/TPMSfinalr...

TPMS doesn't alert until tires are something like 25-30% under pressure, which is pretty severe.[1]

[1]: https://www.nhtsa.gov/sites/nhtsa.gov/files/fmvss/TPMSfinalr... (page 10)

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