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 he did not realize that his lane was
closed, though, until he mowed down a traffic barricade with the front of
his car.
To delete the comma between ahead and he would cause an equally bad problem, a fused sentence. A fused sentence occurs when you have two main clauses joined with no punctuation whatsoever. Many road signs warned Tommy of the construction work ahead ... begins the first main clause. He did not realize is the second main clause.
The spot between ahead and he needs a stronger break than blank space.
You might want to consult the rules for fixing comma splices and fused sentences.