Hah. I wrote a long post talking about this, and then said that my laptop bit the dust yesterday and I hate using the Internet on my phone, and to prove my point it didn't post what I wrote and now it is all gone. Damn.
Long story short, I am almost certain that those economists took pop growth into account. Thinking it through, it also occurred to me that if the model is true, you would expect the old roads to have stayed at the same level of congestion for years despite population growth, because that level was the equilibrium point beyond which people would avoid driving.
Damn you guys are making me want to take transpo planning courses.
Pretty convenient that my "hard drive failed" right when you are about to come out with an OS that my computer can't upgrade to, huh Steve Jobs? "Yeah, the same day I upgraded to the next-newest operating system my hard drive coincidentally died."