Hi Richard,
Thanks for using Google, the information, and the files!
As with the Tunnel, the model is just not really OK. The Bridge has a large N-Gon in it.
The “Spline” polygon object was doable in a few minutes, including improving the UV.
The Bridge would take maybe 5-10 minutes to build, with a clean mesh. With no problems.
Take the concrete material while the (white) material tag on the Bridge is set to anything other than UV. No problems, right? So the freeze that you notice is the application trying to figure out how to solve the N-Gon into UV data. N-Gons are great DURING modeling but not really that great after that.
Let me know if you need that Bridge as a file. I can set it up for you; it is really a simple one. Think of modeling half of the arc, then use a Symmetry, then Clone. Make it editable if needed.
All the best