Ahead Stop
Ahead Stop
[Words are painted in white on a black road with green grass on each side and a gray sidewalk along the road to the left with a single flagstone going of from the sidewalk and out of the frame to the left. The words get smaller towards the top due to the perspective.] BACKWARD. I READ THINK ENGINEERS HIGHWAY

This comic refers to how, in some countries including the US, words or instructions written on the highway are always backwards from how you would read them. It seems that the "highway" engineers write the words as if you would read them as your car goes over them. Sometimes this approach works, other times (probably most of the time) it is terribly confusing. The sentence on the comic is: Highway Engineers Think I Read Backward. Adding the period is a perfectly hilarious touch, as there are probably not too many periods on the highways.
The title text is referring to how the words of the opening sequence of Star Wars Episode IV: A New Hope (released in 1977) move from the bottom of the screen to the top so that it can be read by a normal human being. However, the image text says the engineers initially reversed the text because people were driving BACKWARDS down the highway trying to re-enact the opening sequence, so they started reversing the word order to get people to drive the "correct" direction.
The title of the comic ("Ahead Stop") is also a reference to this phenomenon because the common "Stop Ahead" instruction would be written on the highway as "Ahead" and then "Stop".
This could also be a reference to Top-posting in email threads and online discussion forums, as summarized in the following comedic signature line:
A: Because it messes up the order in which people normally read text. Q: Why is top-posting such a bad thing? A: Top-posting. Q: What is the most annoying thing in e-mail?
[Words are painted in white on a black road with green grass on each side and a gray sidewalk along the road to the left with a single flagstone going of from the sidewalk and out of the frame to the left. The words get smaller towards the top due to the perspective.] BACKWARD. I READ THINK ENGINEERS HIGHWAY
