This plot of Nextel site locations is from data I manually gathered with a Nextel phone from spring 2005 through 2007. Each tower is a blue dot. In Detroit along 696, some of the Nextel sites are only about 1/2 mile apart. And still in the evenings I would have problems making calls due to all channels on the site busy.
Obviously these are just the locations I drove around to for work and meetings. I didn’t record out of state because it would have gotten overwhelming, and I knew even in 2005 that Chicagoland was swamped with coverage.