This is from Shentel's recent earnings call:
"As I have discussed before, our wireless customers are twice as likely to churn if they live in a county on the parameter [perimeter] of our network
The reason is that for many of our customers their community of interest is outside of our network. So, they often travel into the Sprint network areas, they don't have the quality of service that Sprint provides in the urban areas."
This is very true where I live which borders Loudoun County - where Sprint signal goes to die. (Clarke, and Frederick are in Shentel's footprint).