Take a look at the original sentence again:
Many road signs warned Tommy of the construction work
ahead, he didn't realize that his lane was
closed, though, until he mowed down a traffic barricade with the front of
his car.
You chose to fix it the correct way:
Many road signs warned Tommy of the construction work
ahead. He didn't realize that his lane was
closed, though, until he mowed down a traffic barricade with the front of
his car.
Many road signs warned Tommy of the construction work ahead is a main clause. He didn't realize is a second main clause. You are always correct when you use a period between the two main clauses that create the comma splice.