Sophisticated cities maintain a model (commonly called a pavement management program) that lets them predict future maintenance cost across the city.
He misses the fact that Ames does get tax money for street maintenance [1] and reconstruction [2]. His estimate for a properly maintained street is quite low -- other Midwestern US cities are able to get 60-70 years from their streets.
He also misses out on special assessments as a source of revenue -- which is very common for residential street projects.
[1] http://www.cityofames.org/home/showdocument?id=22486 page 178 shows state taxes as a revenue source
[2] http://www.cityofames.org/home/showdocument?id=8045 page 91/92 shows a variety of sources, including state taxes