While I'm still a novice Linux student, my travel time for work had increased dramatically in the last year as an IT installation tech. I've noticed a great deal of discrepancy between my GPS speed readout and the speedometer on my car. This has in general been a positive for me as I have some shall we say "rapid deployment issues" historically.

Since I drive a lot more, I've been working on changing my driving habits and on highways I often use cruise control. I'm wondering (with the understanding that Android version as well as software and software iteration concerns will effect the actual validity of the argument) has anyone tried introducing data from GPS software to battle traffic/moving violations in court? If so, in what states, and what have been the outcomes?

If this HASN'T happened yet, then I guess I better take this post down quickly and get my butt in gear learning to code:)

..but I would imagine that somewhere, there must be instances of this.


  • I suspect that any effort to do this would be hamstrung by the fact that GPS measurements are typically only accurate to within 5 meters. This means that if you're reading your position out every second, the velocity numbers you'd infer from it would be accurate only to about 5 m/s ≈ 10 mph. You could average them over a longer period to get a higher accuracy (and I suspect that's what most GPS devices do), but it'd be easy for a cop to argue that you slowed way down when you saw him such that your average speed over (say) a 10-second period was below the limit. – Michael Seifert Oct 30 '19 at 18:28
  • So it seems like an instance wear the tech isn't there yet. Police can actively try and track a single metric so the defensive requirement for the tech's dataset is really high. My understanding is that a lot of the variance in GPS measurement comes from what amounts to a sample size; the number of times per unit (seconds) the GPS threads a sample. If we had (access to) GPS devices with more processing power, maybe satellites with more bandwidth and deliberately record all the data on a trip, it seems like that could be translated to use in proving a rate of speed in a given time frame? – bobbymobetta Oct 30 '19 at 19:24
  • Hello bobbymobetta! Welcome to Law.SE. Please read the tour page, linked at the bottom of this page. – isakbob Oct 31 '19 at 14:20
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    @MichaelSeifert As I understand, GPS velocity measurement is based on the doppler shift of the signal from the satellite, not displacement divided by time. – HAEM Oct 31 '19 at 14:39
  • Prior to the 2000s, commercially available GPS would also register a further variation to actual position by design. The satellites were deployed for military navigation, and for security reasons, civillain devices were less than ideally accurate. One of the last acts of Bill Clinton's presidencies was to disable the device causing civvie GPS to be less accurate. – hszmv Oct 31 '19 at 17:19

Not exactly, but something similar has been tried (and successfully so) in the UK

The device wasn't an android phone though but rather a dedicated GPS-based driving telematics device.

We don't know for sure whether his evidence would have been accepted though as in this case when the accused said he had evidence from the telementics device the prosecution declined to present any evidence. As his solicitor put it:

"Based on previous experience in other cases, laser speed detection devices can produce erroneous results and in this case Mr Herron was convinced he was not speeding as alleged and stated he had telematics data to support that view. When confronted with failings in court the prosecution determined to offer no evidence."

This doesn't really set a legal precedent that "GPS beats Laser speed gun" though - because the CPS chose not to present evidence.

As others have commented though there's quite a few potential pitfalls with doing this - GPS data carries a certain level of uncertainty, it's pretty accurate these days, to quote GPS.gov:

GPS-enabled smartphones are typically accurate to within a 4.9 m (16 ft.) radius under open sky. However, their accuracy worsens near buildings, bridges, and trees.

and specifically for speed measurement:

As with positioning, the speed accuracy of GPS depends on many factors.

The government provides the GPS signal in space with a global average user range rate error (URRE) of ≤0.006 m/sec over any 3-second interval, with 95% probability.

This measure must be combined with other factors outside the government's control, including satellite geometry, signal blockage, atmospheric conditions, and receiver design features/quality, to calculate a particular receiver's speed accuracy.

The thing is there's going to a big range of accuracy in what a phone can offer - does it have A-GPS, GLONASS, how good is the GPS antenna, where is it positioned in the car etc etc. A lower end device shoved in the driver's is going to offer significantly worse accuracy performance than a top end handset sat in a cradle with clear line of sight to the sky.

In short it's a long way from being some form of get-out-jail-free card you can use in defense of speeding allegations. And it certainly gets it wrong often enough that you wouldn't want it (at least not in it's current form) to be the final arbiter. More than a few people have been dinged by their insurance companies when the supplied GPS tracker has accused them of driving beyond the limit when they haven't!

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I don't know of anyone who has tried this, probably because it would be meaningless.

Citations are based off an officer tracking your speed, not what your speedometer or GPS says, so it would not be relevant in any way.

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  • I'm trying to present a situation where use-gathered challenges data presented from a radar gun which are rife with inconsistencies, easily knocked out of calibration, subject to user errors and also simply lied about. Using a "radar detector" sort of seems like too little too late and I understand are increasingly quasi-legal. I'm wondering if and when we will have ability to police ourselves, or at least be able to state with confidence "What seems to be the problem officer? Because I know I wasn't speeding!" ; ) Not sure why you'd say meaningless. – bobbymobetta Oct 30 '19 at 19:33
  • The same issues you say arise from radar and such arise from gps on your phone though. – Putvi Oct 30 '19 at 19:34

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