Here's how AT&T described the accuracy of historical cell phone location data in an amicus brief on the issue in November 2014:
The precision of this location information varies according to the array of the towers and technology employed. As the density of the cell towers increases (decreasing the area covered by any particular tower), the precision of the CSLI increases correspondingly. Rural or sparsely populated areas generally have fewer cell towers, each serving a larger territory. In more densely populated areas, towers are much closer together and serve smaller areas, generating more specific location information. As customers demand more bandwidth to support smartphones, video services, and other high-volume Internet access, service providers are increasing the density of cell towers, further shrinking the size of particular cells. Service providers are also increasingly boosting their network coverage through small cells known as “microcells” or “femtocells” that may cover an area as small as a single floor of a building or an individual house.
Cellular communications technology may also generate other, more precise forms of location information. For example, some mobile devices, such as smartphones, are equipped with GPS technology which determines the device’s exact location based on signals received by the phone from a network of satellites. In addition, because mobile devices are often in contact with more than one cell tower at a time, it is often possible to locate the device through triangulation – i.e., determining the point of overlap among the areas covered by each of the multiple towers within range of a particular device. ...
CSLI at times may provide more sensitive and extensive personal information than the car tracking information at issue in Jones [ed. note: a SCOTUS decision declaring use of GPS trackers is a search]. Users typically keep their mobile devices with them during the entire day, potentially providing a much more extensive and continuous record of an individual’s movements and living patterns than that provided by tracking a vehicle; CSLI, therefore, is not limited to the largely public road system or to when the device user is in a vehicle. That difference, in turn, may enable officials to use historical and prospective CSLI to construct a more detailed and intimate portrayal of the targeted person’s daily habits and work and leisure routines – including activities related to the home.If "historical data is inaccurate" is the best argument they've got (all law enforcement witnesses granted that real-time tracking should require a warrant), this bill with its 97 House authors ought to do fine!
The whole AT&T brief (pdf) is quite a remarkable document for those interested in the topic. It staked out the corporate arguments in a systematic and compelling way for installing a probable-cause warrant requirement for government to access this detailed personal information.
MORE: It's worth noting both AT&T and Verizon registered in favor of Hughes' bill at the hearing.