This discussion is a duplicate thread.
Galactus responded to this discussion at the other location, with an interpretation of the phrase "the arch must provide a minimum clearance of 4m over the highway" that is better than what I described in my post above.
Galactus reads it as a 4-meter clearance at the lowest point directly above the outside edges of the road; I was reading it to a 4-meter clearance at the highest point (above the center of the road).
I believe that Galactus is right, and I was wrong. Therefore, most of the information that I posted above is wrong, too.
Please go to the other location HERE, and read Galactus' post.
Oh, it looks like Elizabeth got it right, too. Credit goes to her, also.