Not very accurate, says Douglas Starr over at The New Yorker. An excerpt:

If I make a cell call from Kenmore Square, in my home town of Boston, you might think that I’m connecting to a cell site a few hundred feet away. But, if I’m standing near Fenway Park during a Red Sox game, with thousands of fans making calls and sending texts, that tower may have reached its capacity. Hypothetically, the system might send me to the next site, which might also be at capacity or down for maintenance, or to the next site, or the next. The switching center may look for all sorts of factors, most of which are proprietary to the company’s software. The only thing that you can say with confidence is that I have connected to a cell site somewhere within a radius of roughly twenty miles.

This isn’t exactly news for those following the issue, but it’s worth remembering — both for those weighing the significance of such records as evidence of crime and for those considering whether collection of the records amounts to a Fourth Amendment search.