I've crossed that line frequently for 40 years and have not seen it.
Highway 40 through St Louis....Something about “more than 2 cartons considered smuggling” or something along those lines.
The law in IL is “each” person coming into the state of IL can only bring 2 cartons of out of state cigarettes into IL.
(now I have never heard of anyone being busted for having a few extra out of state cartons on them, but my guess is if you get pulled over for speeding and act like an a$$ when you have a few extra out of state cartons with you then you might get nailed for it along with a speeding ticket)
Also there are signs at my IL/MO crossing posted about smuggling cigarettes into IL they stand along side the No Jammers allowed in the state of IL signs.