"In 1935, city limits were clearly identifiable," wrote the Science Foundation. This is no longer the case, and many urban areas snake into valleys and along transit routes.I also imagine a 20 year moratorium on growth would not go over so well here. Though I think here it's called a growth boundary.
Researchers are proposing benchmarks to limit urban sprawl. In August 2008, a coalition of environmental organisations submitted more than 110,000 signatures to the government in support of a 20-year moratorium on new growth.