Early Millimeter Deployment Might Remind You of Early 1G

This map of Verizon’s 5G deployment in downtown Denver illustrates the use of 5G as an overlay on 4G for capacity reinforcement. The map also illustrates a feature of millimeter wave spectrum when used to support 5G or other networks: it does not propagate as well as mid-band or low-band frequencies.

Note the coverage basically outdoors, along streets and sidewalks. For some of you, that might harken back to the early days of analog, first-generation cellular service, which essentially was a "call from your car" service. That being the case, it made sense to build first along major streets and highways.

Note that 4G coverage is shown as pervasive. That illustrates the importance of non-millimeter wave frequencies to supply coverage, especially indoors. 

source: Verizon