Take another look at the original sentence:
You wanted to fix it this way:
In this sentence, you have two main clauses. The first one is Many road signs warned Tommy of the construction work ahead, and the second one is he didn't realize. A coordinating conjunction like but can connect two main clauses if there is a comma before the conjunction. This choice is wrong because no comma precedes but.
You might want to consult the rules for fixing comma splices and fused sentences.