Capacity planning? We've heard of it...
Communications companies just figure out what the average data transmission need is over 24 hours, and then plan to meet it ... on average.
I live in Louisiana, about 70 miles inland from New Orleans and we had quite decent cell phone service until Hurricane Katrina force everyone out of New Orleans - our city's population doubled overnight and the cell phone service crashed, for a month it took about 10-20 minutes to get a phone call through, text messages would normally get through within an hour.
Sure, this was exceptional but it illustrates that when you live in a city and a lot of new phones arrive then the phone service is going to slow down - in cell tower usage terms, what do the companies consider to be adequate capacity?