You're right of course.

Blackwall seems also to have two sets of lights and barriers, and an off-ramp in between. That's probably for fire safety too to close and evacuate the tunnel and get the emergency services in, but I imagine it's used for height detection too if a loud CLUNK on your truck cabin isn't enough.

As an aside, the person who signed the original heights as (13ft)(4m)(9ft)(2.8m) needs to learn a bit about UI design. Yes, two lanes, but the gap between the central two signs is far smaller than to the other sign for the same lane. Also 4m is just over 13 ft 1 inch, which there'd be space to include as there's already a 0 on the leftmost sign (and from the rightmost we see that decimals are allowed on signs). Guess we're going to rely on the CLUNK after all.