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Trip Max Speed - 153 MPH?

marfa22 0 Points
edited November -1 in GPS Discussions
I was stopped by state police for speeding; I was approximately 1/3 through my 150 miles trip. The police officer touched my GPS to see the trip info. When he saw the trip's max speed of 153MPH, he gave me the ticket, saying that somebody was really ... speeding in this car.

I'm pretty sure I wasn't going 153MPH at any point in this car (and in any other).
Does anybody know how the max speed is calculated?

Comments

  • Tim 1481 Points
    No offense, but I'm finding your story a bit difficult to believe. Regardless, speed is calculated with elementary school math... speed = distance /time

    The GPS records your position about once per second (the time component) and the distance is calculated by your GPS position at one point compared to your GPS position at the next point. (Each point would be about one second apart.) However given that consumer grade GPS devices have an accuracy of +/- 10 meters, those two positions could theoretically say you traveled 20 meters further in a second than you actually did... or even more.

    I went hiking once and a Garmin device said my max speed was over 20,000 mph due to a signal anomaly. They are not a terribly reliable source of max speed.
  • lordgrinz 0 Points
    Wow! And I thought 3mph with a 40lb pack on my last trip down the AT was fast! :wink:
    I went hiking once and a Garmin device said my max speed was over 20,000 mph due to a signal anomaly. They are not a terribly reliable source of max speed.
  • Tim 1481 Points
    I was trying to keep up with my sister and she hikes like a mountain goat... but I'm pretty sure I wasn't going quite that fast. :)
  • marfa22 0 Points
    I had 2 kids in the car - I wouldn't even think of going that fast... and it wasn't even a highway, just a country route.

    I've never reset the trips info. Could this be the case? Although, I commute from metro area and with traffic jams, my max speed should be even lower than actual.
  • Tim 1481 Points
    I'm not saying I think you were going that fast... just seems strange that the officer would want to see the GPS, or that he even would have a right to do so. (Although my background is in GPS, not civil rights. :))

    You can easily get your GPS to read several hundred mph on that screen... just go for an airplane ride.

    I just did some quick calculations because I was curious. GPS is accurate to within 10 meters, 95% of the time. Let's assume we were on the "good" side of 95%, but we still had the worst case scenario that in one second the GPS was off by 10 meters "behind" us and the next second the GPS was off by 10 meters "ahead" of us.

    In that case, the distance reported is 20 meters greater than actual. So 20 meters per second "off", which is about 45 mph. So if the GPS actually had this type of anomaly (which is unlikely) the GPS would report a speed 45 mph greater than what the person was actually traveling.

    As I indicated above, the GPS can be off by way more than that... but I thought it would be fun to calculate the worst case scenario based on being within the 95% accuracy standard.
  • marfa22 0 Points
    looked into different forums - problem does occur.

    Per one person's statement, just for the info: "Contrary to popular belief, a GPS does not constantly track your position but rather records a specific location point at selected intervals. Points are usually collected more often on a winding road versus a straight one (if point collection data is set to automatic). Your track log is then displayed on the screen as the GPS "connects the dots".

    Speed is calculated by the amount of time it takes to get from one specific point to another (in addition to the distance). If your satellite signal is temporarily weak or disturbed it is possible that the GPS will trick itself into believing it jumped from its last recorded point before disturbance to its present position in a shorter than realistic time, therefore creating your pseudo max speed."

    Thought it would be interested to know..
  • Tim 1481 Points
    looked into different forums - problem does occur.
    Which is almost exactly what I said above. :) But it isn't correct that the GPS changes its amount of data collection based on the curvature of the road-- it uses either a set time interval or a set distance interval. It doesn't care about curvature.
  • marfa22 0 Points
    yes, thanks for the ideas. will reaserch further to fight in traffic violation court, as the police officer put that fact (Max speed on GPS=153) in writing to support his position.

    with regards to his rights - I didn't even have time to blink whe he was already in my GPS helping himself, without my permission...
  • Tim 1481 Points
    I don't know anything about what your rights are, etc.... but it seems as though the only thing he would be able to prove would be what he got on radar. After-all how would he know you didn't take a trip on a plane with the GPS?

    Keep us posted how things turn out.
  • K9CHP 0 Points
    I'm not a lawyer but it reeks of illegal search and seizure.
  • yooper1 0 Points
    MAN !!! What kind of car you got ???? Short of an expensive sports car,You will find a hard time getting a normal car to that speed as they are electonically limited to a max speed,My F150 will cut out at 103mph,and BIL Chevy PU at 96mph
  • Mark29 0 Points
    MAN !!! What kind of car you got ???? Short of an expensive sports car,You will find a hard time getting a normal car to that speed as they are electonically limited to a max speed,My F150 will cut out at 103mph,and BIL Chevy PU at 96mph
    I was going to suggest the same thing. This is definitely worth fighting. It shouldn't be difficult at all to track down whether or not a given vehicle has an electronic limiter on the speed. Also, I don't see how a consumer gps, with no calibration records proving it is accurate for speed, can be used as a basis for submitting a ticket.

    Mark
  • JohnBoyToo 0 Points
    MANY tuners can disable a limiter.....
    not wise unless u have speed rated tires, etc....

    but, agree -"fightable"
    :twisted:
  • tdekany 81 Points
    I would be very shocked if what the officer did was legal.

    Also as others have mentioned, what kind of car were you driving? If it doesn't have a built in nav system, your car can't go that fast anyway. :lol: :lol: :lol:

    Please let us know of the outcome.
  • I would seriously doubt they could be used to prosecute...:roll:

    You’ve probably seen sporadic news reports about people using their car’s GPS to fight their speeding tickets. Will it work? One of the most famous cases centers around a Sonoma County teen who has been fighting his speeding ticket for over a year. His parents have hired a GPS expert to testify in traffic court as to the accuracy of their son’s GPS unit. And the case is still trudging through the courts. I saw a simple version of the GPS defense in Spotsylvania County General District Court the other day. A gentleman went up to the bench for his speeding ticket hearing. The Virginia State Trooper testified as to the defendant’s speed and his radar accuracy. Then the defendant claimed, “but my Garmin said I was only going 78.” The traffic court judge was not impressed. He explained that the defendant had no proof of the GPS unit’s speed reading and the GPS could very well be wrong. The ruling? Guilty. To effectively use a GPS defense in traffic court, you need two things: 1.) proof of the GPS speed readout at the time the police claim you were speeding, and 2.) proof that the GPS was accurate. Some types of GPS units provide logs of the readouts and activity. But proof of the GPS accuracy would probably require a GPS expert, at least until traffic court judges better understand GPS technology and actually put faith into it. The conclusion? You need more than “my GPS said I wasn’t speeding” to get out of your traffic ticket.
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