Who controls the signing keys and the whole signing process? What about key revocation if someone steals the key? Will a municipality in Texas really be willing to not be allowed to create a new stoplight without approval from the federal agency in charge of the keys? What’s to stop someone stealing a “real” stoplight from bumfuck nowhere and putting it in the middle of the 101 at rush hour? What about replay attacks? What about signal jamming? Etc. etc.

You raise some genuine possible concerns but generically I'd probably tend to just say, "laws will stop them" just like they already stop someone deliberately endangering lives by placing a real fake stop light in the middle of the 101 at rush hour.

The real concern would be whether someone can engineer a terrorist level mass scale attack but as long as it requires physical tampering that adds up to a tremendous amount of work. So if the signalling is largely burned into fixed infastructure it eliminates a lot of that or at least sets the bar high enough that its probably more work than various other types of attack that are likely to be just as impactful.