The tandem bike makes the whole joke. A normal bike would have been meh, but this is much funnier, because you wonder about the riders and their relation
It definitely made me wonder what the point of a tandem bike was to be included.
Then I thought it might have been funnier if the first rider was still on the bike while only the second rider hit the bridge, presumably because his view was blocked by the first rider who was able to see the bridge and duck in time to miss it.
I received a really nasty blow right to the front of my forehead like this as a kid. Going through a little concrete tunnel on a tricycle. Except whereas I normally had the low trike, this time I had the tall one. Smacked my forehead straight into the concrete.