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 chose the correct way to fix the error:
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.
Many road signs warned Tommy of the construction work ahead is a main clause. He did not 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.