In the real world, tracking folks via their phones is less about triangulating signals before a killer hangs up and more about digging up cell phone records during the investigation phase. Cell phone towers can track the location of your phone to within a two-mile radius. That might not be accurate enough for them to pinpoint the Parkour Bandit and embark on an awesome rooftop chase, but it's more than enough to potentially ruin an alibi.
How It Can Go Horribly Wrong:
Cell phone tracking is widely used as evidence. In 2013 alone, U.S. courts saw a staggering 37,839 subpoenas, warrants, and court orders based on it. Which is kind of unfortunate, since cell phone tracking is inaccurate. So very, very inaccurate.
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"We got the perp. He's crossing into Egypt."
"Didn't this guy jack a car in Queens?"
Anyone who's dealt with a telecommunications company knows that cell phones are a huge clusterfuck at the best of times. Tracking via cell phone records is no exception. Your phone's not always going to choose the closest tower to have the best possible signal. The towers don't always have the same range, and there's plenty of overlap.
For an example of how this can go wrong, look at the case of Lisa Roberts, an ex-lover of a prostitute who was strangled in Portland, Oregon. Her cell phone data had "pinpointed" her in the area before the crime. She was forced to take a plea bargain of 15 years rather than a full 25 years in prison, and was only exonerated after serving 12 years.
"So, 'whoops' on that one ... How about we get you an iPhone and call it even?"