DUH!

That is absolutely outstanding work!  YOu have capture the proportions of a modern U.S. Interstate bridge very nicely.  One questions - what color(s) did you use for paint?

Paint

Philip,

as mentioned in the tutorial:

"After some testing I decided to paint the roadways with a mixture of one part Tamiya XF-19 sky gray and two parts Tamiya XF-2 flat white. A week later I painted the remaining areas of the bridges, the piers and the already installed abutments with a mixture of one part flat gray and four parts flat white. Furthermore I accentuated the roadways with gray artists chalk."