Whoa, cool video!!! That caught me off guard too when the bridge and tunnel elements just sprang up like that...

As Michael said, the GH developer has basically coded a parallel version of Rhino commands and script functions. But Houdini was built as a nodal system from the ground up, where everything can be accessed by procedures. So GH functions do not go as deep as that yet. It may never reach that kind of depth... But this video is only showing procedural geometry and not using any special deformers or animation effects, so I think the entire project could be duplicated in GH. GH can also reference standard Rhino objects. So a curve could be used to define the road, then move the curve's points and GH will redraw all the geometry based on that curve. And then account for where the curve intersects the ground mesh, it's height relative to the ground, the length of spans for each elevated section, etc... It must be a tremendous amount of work using either GH or Houdini. I wonder really how much time was spent to set this up...

I've used GH to model a few parametric objects, and you almost have to forget what you know about modeling. Then start to think what is really happening within the program which allows you to model each element? How does an object get replicated along a curve and maintain orientation related to the curve? then you have to start dealing with planes, vectors, frames, domains, lists, etc... and the list seems never-ending. Even something simple like extruding an edge by a specified length requires a lot of steps to set up... It will give you a whole new respect for programmers and those GUI's which they create to do all the heavy lifting for you...