Review the original item:
Many road signs warned Tommy of the construction work
ahead,
he did not realize that his lane was closed, though, until he mowed
down a traffic barricade with the front of his car.
You wanted to fix it this way:
Many road signs warned Tommy of the construction work
ahead but he did not realize that his lane
was closed, though, until he mowed down a traffic barricade with the front
of his car.
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 did not 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.