GPS Used As Defence In Radar Speeding Case

James Thigpen writes "There is an article over at Ars Technica about an accused speeder contesting his speeding ticket based on his car's built-in GPS system's records. According to the article his car says he was going slower than the radar gun clocked him at. Contesting a ticket based on GPS data has never before been tested in court."

Video Evidence

I've often thought it would be a good idea to have a constant video recording your driving, like the police camera setups. This could help clear up who to beleive at the scene of accidents, because of the video.

Plus it would be cool to have onboard footage of your driving for analysis and review.

Re:Video Evidence

If you dragrace with yourself and yourself alone one a lone road in the middle of nowhere, does it really matter? I would not like to have the authorities to have a closer look at my driving. I hate the speed cameras they tend to set up everywhere on the road, but in front of schools for instance (where they'd really matter), I'm yet to spot one.

Re:Video Evidence

Yep, police work is only performed where it matter$, aka speed traps and deliberately low limits. Saving lives is not a profitable business, which is why no matter what you do (or don't do), if a cop shows up, you get a fine.

In my opinion, if they're not enforcing speed limits in the few areas where they are actually beneficial, then we should abolish that system entirely as it is working for no one. I pay taxes like (most) everyone else, if that money isn't enough to afford proper police without the need for profiteering practices, then raise my goddamned taxes and destroy those stupid radar guns. Maybe then people will start respecting these so-called peace keepers again.

Something is very very wrong with the world when honest law-abiding citizens live in fear and/or contempt of the law.

Re:Video Evidence

Sure sometimes radar guns are inaccurate but its also true that people speed and speeding is highly dangerous.

Imho the latter outweighs the former and radar guns are generally a good thing.

Re:Video Evidence

Speed limits are arbitrary, and (specifically on the highways between Brisbane and Melbourne) designed to make money, not save lives.

There's a simple solution to that - if you don't exceed the speed limit, they won't make any money from the cameras. So, if they are about making money, then they would be removed once they stopped being profitable. Unfortunately, it seems highly unlikely to get the majority of people to obey the road rules for even one day or one week - so it looks like the cameras are here to stay. I still think it would be a hilarous protest, though - everybody obey the law today, and screw the police and government. It would be an act of civil obedience.

Re:Video Evidence

I don't think so.

Kind of right. What they actually do is reduce the tolerance levels. This happened in Victoria (Australian state - probably has some of the most brutally enforced speeding laws *in the world* - unsurprisingly its roads aren't meaningfully safer), where you will be booked for driving as little as 3km/h over the posted limit (how this lines up with speedometers only having to be accurate within 10% hasn't been tested in court yet AFAIK).

So whereas you use to be able to do 120km/h in a 110 zone without too much to worry about, now you'll get pinged for 114km/h. No-one without an agenda seriously thinks a ~3.6% speed differential has any meaningful impact on road safety.

Police and politicians have to get places by car, too.

Poor examples. Police can (and do) break the speed limit at will with little fear of either detection or punishment. Politicians are typically being driven, for short distances, and only in urban traffic.

Generally I think the speed limits are pretty reasonable. It's just that drivers can't stand any form of restriction, and always want to go faster.

Also untrue. Research has demonstrated that in typical conditions - especially high speed roads like motorways - drivers choose the safest speed for the conditions. People actually interested in road safety know this as the 85th percentile. It's what the posted limit on a road *should* be set at for "maximum safety" (but usually isn't).

For a concrete example, there is a major highway north of Brisbane, Queensland (2 lanes each way, divided, limited access). Some years ago the speed limit was *raised* from 100 to 110km/h (amidst the typical outcries from ignorant fools about how the roads would be awash with blood). Not only does the road remain as safe as it was, but average traffic speed actually *dropped* by about 3km/h.

Seriously, if people can't follow a simple speed limit, why should they be entrusted with more liberty on the road? If people would obey them and drive like sane people, then they could be allowed to drive faster. You have to earn responsibility.

Because following a badly set speed limit - *especially* on higher speed roads like motorways - actually *increases* risk. *DRIVERS* have to earn trust ? What a joke. Maybe if the government was more interested in saving lives than making money - and demonstrated it - we'd be able to trust them with things like speed limits.

Very, *very* few governments have shown any real interest in improving road safety. Why would they ? Doing so would be expensive (both in monetary and political terms) and it's trivial (and cheap) with a good advertising campaign to demonise things like speeding (despite it being a relatively insignificant factor in overall road safety) so they have someone to pin all the "carnage" on.

Re:Video Evidence

Speed limits are often based on the quality of the road (of the road surface, of the ability of drivers entering the road to see cars coming, and of the ability of drivers on the road to see hazards on the road ahead of them (reduced by curves, for instance)). The road surface quality degrades with time. Occasionally, studies are done in specific areas that demonstrate that the speed limit really is too high for the visibility of the road, both for drivers entering the road and drivers on the road. Other times, the population increases, and thus so does traffic. In each of these situations, reducing the speed limit is the appropriate immediate step. Further steps might include resurfacing the road, widening the road, etc.

Yes, speed limits are often arbitrary and designed to trap drivers. But claiming that speed limits are never related to safety is foolish, and claiming that speeding is not at all dangerous is also foolish. Higher speed increases both your reaction distance and the severity of any mistakes. Increasing either of these reduces safety.

I'm glad I don't have to share the road with you.

Re:Video Evidence

Actually, some college students at Georgia State University tried an experiment [google.com] in which they blocked off all lanes on Interstate 285 going 55 miles per hour, the speed limit. Keep in mind that most people drive 65 to 70 on that road.

As a result, the people behind them got very angry and began active extremely dangerously. One van even had an accident when he passed them on the right shoulder and clipped a car that was parked in the emergency lane.

There is nothing inherently dangerous about going faster than the speed limit. Sometimes, when it's raining and there is low visibility, driving the speed limit is unsafe. Other times, when there is low traffic volume, high visibility, and the roads are dry, it's perfectly safe to go 10 to 15 miles per hour above the limit. The law doesn't take that into account, though, and as a result, the speed limit is set arbitrarily low on almost every road.

Re:Video Evidence

Problem is, in an age where insurance is a requirement people think that lowering insurance rates is an appropriate goal for public policy. Drag racing, even by yourself on a deserted road, is risky behavior, which raises risk for insurance companies and therefore their rates as well. They're not just going to absorb that loss.

Re:Video Evidence

It would have to be hidden. A lot of cops will make you turn off any recording equipment they see as soon as they start talking to you - and if you don't comply many of them will just arrest you for BS charges. Not saying ALL cops, but I've read articles where already asshole cops went berserk over recording equipment and the person not dropping their pants and bending over to their demands.

Re:Video Evidence

I'm pretty sure that's not what the OP is advocating. If you set up a camera in your car, YOU control it and all the tapes. If you do something illegal or that you don't want taped, you can either turn the camera off beforehand or destroy the tape after. The only place the government comes into this is if you turn the tapes over to the government/court to prove your innocence.

This could only be the first step

If this ends up being a valid way to argue against getting a speeding ticket, the next step I see will be people speeding like hell, and then hacking their car's GPS records to show they were going at the speed limit.

Re:This could only be the first step

I have heard stories of people trying to submit GPS data logs in the past to prove they were not speeding. The judges would not accept the data because it could be considered suspect, particularly if you presented it on a USB dongle since the data is beyond easy to modify. Radar does have its issues, specifically if you are in a group of cars (have you ever been blamed for speeding when the car beside you was passing you). Unless you can provide data in a method that is deemed "un-crackable", I doubt it would be allowed. I could easily re-run the route that I was on when I got stopped, take the track log and modify the time stamps (if they are present and that depends on the GPS data stream you selected). You would want the time stamps to be there to compare against the time the police officer stated on the ticket. I have to take this jab at the judicial system though, despite the fact the the citations say you are not pleading guilt or innocence at the time of the infraction, you are pretty much labelled guilty, the police never lie in the courts point of view and if you claim you are innocent, you get treated as if you were guilty anyway. The only way I can see to defend yourself is to have the same set up in your car as the police do and have it display speed on the recording. Then again we saw recently what happened someone who tried that in Missouri.

Re:This could only be the first step

It reminded me of this case [bbc.co.uk] from earlier this month. The inventor in the story was testing his new device when he was clocked by a speed camera. In the court case his GPS logs were used as evidence that he was 12mph slower than the speed gun recorded. He may have had a motive for pursuing it through to a court case as he is starting a company to comercialise the device.

Re:This could only be the first step

the next step I see will be people speeding like hell, and then hacking their car's GPS records to show they were going at the speed limit.

Yup. On most people's home projects list that's the one right after "Get microwave to stop blinking 12:00."

Open source GPS?

But will he be able to produce the source code for the GPS when the police request it to check its accuracy?

Breathalyzer Source Code Revealed [slashdot.org]
Closed Source -> Charges Dismissed? [slashdot.org]

used in Taiwan

GPS data was actually used recently in taiwan to prove a man's innocence. A truck driver A went into an accident with a motorcyclist B. A stayed and helped B up, and even paid cash. B said he's fine, so A drove off, only later to recevie a notice that B has filed a hit and run case against him. The court found A not guilty since the gps data showed that A stayed at the site for more than 15 mins.

Brings accuracy into question

The pretty large difference between his 'radar' speed, and his 'gps'(actual) speed was pretty large. IMHO this sets brings into question just about every speeding ticket ever given by radar gun.....

lets say that the gun is wrong 1% of the time, which in the case of a cop handing out tickets by hand is okay (imho) because there is human intervention, he (or she) can look at the thing, bang it on his hand a little, and shake the error off as a fluke.
The speed cameras on the 101 in scottsdale, arizona issue about 250 tickets daily. Thats 2.5 tickets daily that the gun gets wrong (the 1% figure was pulled from my ass, but I'm using it as an example). With THIS there is no human intervention at all (other than a pissed off commuter)..

grr...not sure where i'm going with this, I just REALLY hate it that humans are being taken out of (at least that little part) of the legal system. I don't want my fate decided by a computer!

Re:Certainly does

Cameras and speed displays don't have radars in them.

They use induction loops buried below the road, and work exactly the same way you do - compare times at positions A and B.

Radar and GPS were in check in California

When we were on vacation in CA, we were stopped for speeding on highway 299 and had the GPS running. I told it to stop tracking the rest of the trip so I can get the data later. When I looked at it, it was dead on what the officer clocked us at so I think this person has a good case.

Testing in UK court case and GPS won

This was tested in a uk court case and the ticket was cancelled

http://news.bbc.co.uk/1/hi/england/wear/7033353.stm [bbc.co.uk]

Are you serious?

Contesting a ticket based on GPS data has never before been tested in court."

Yes it has...read up. The success rate, though, is the same as the rest of the cases. The majority of whom only get off because the cop that pulled them over never show up in court.

Speed = Distance / Time

From my understanding, and the contention of the officer, the GPS logs average speed. Which means that during a short period of time, the defendant could have greatly exceeded the speed limit (and was clocked by the officer at that time), while the average speed was far lower than that. In which case, both the cop and the defendant are correct, and the cop is till valid in giving the ticket...

Re:Speed = Distance / Time

Two things:

1. What is the time the GPS device averages over? On the devices I've seen it updates about every second. Unless you have a REALLY nice car you're not going to go from 65 to 90 and back down for long enough to average 65 over that kind of time.

2. At least one state (MA) and perhaps others have laws that require your AVERAGE speed over some distance (I believe MA is 1/4 mi) to be over the limit for a speeding ticket.

Re:Speed = Distance / Time

First, when the GPS unit itself calculates the speed, it records your instantaneous velocity, not an average. It calculates this using the doppler shift present in the GPS signals picked up by the unit, not from how far the unit has travelled.

Let's go over some basics:

a) There is no such thing as "instantaneous velocity" - as velocity is a function of time.

Corrolary: You can /approach/ t=0, but the closer to "near-instantaneous velocity" you try to measure, the more accurate your measurements must be - alternatively the higher the margin of error will be.

And the problem with the radar/lasar guns is indeed that, because they try calculate "near-instantaneous velocity" they are very *very* susceptible to error, particularly at the ranges the police often try use them at (hundreds of metres).

b) Noticing a doppler shift in waves from a (relatively) stationary source would require that you have a non-zero velocity relative to the source (ie the distance between you and the source change). I'm reasonably sure this velocity would be immeasurable from a consumer car in a GPS over a short period of time and, further, that any measurable doppler would be due far more to the /satellite/ moving, not the car..

I.e. I havn't done the calculations (it's not just linear, cause any doppler will be induced by the curvature of the earth, not directly by the car's speed), but you're talking about measuring doppler due to /millimetres/ of movement (curvature of the earth), as a car moves perhaps a tennes of few metres. It's beyond believable we could measure that with any useful accuracy in a car.

So I call bullshit, unless you show me the numbers to prove otherwise.

Instantaneous velocity

No, my friend, you are wrong.
You see, Instantaneous velocity is the limit of the average velocity where the time of averaging tends to zero.
In other words, the value of f'(t0), where the position x is x = f(t) at a given time t0.
Or in other words, angle of the tangent of the curve x = f(t) in the given time t0.

Now, if your argument is that "a GPS device cannot give the measure of the instantaneous velocity because it does not sample fast enough to get a really good approximation of the curve x = f(t) and hence, the value of f'(t0)", then you could be right because 1Hz is not really a high sampling rate. But you could have said so ;-)

The (analog) speedometer in most cars measure speed by measuring the RPMS of the gear box and multiplying by gear ratios and tire size: they normally do that with a continuous measuring (springs and coils), and what they measure is a good approximation of the instantaneous velocity of the vehicle. A good analog speedometer is somewhat reliable, especially if the scale is correct(*)

(*) their scale is not linear like you see in a normal car:

0 .... 20 .... 40 .... 60 .... 80 .... 100

but exponential, so it should be like:

0 . 20 .. 40 .... 60 ........ 80 .............. 100

and this is why they have a "sweet calibration spot" (normally near the top of the dial; have you already thought about why they make 1.2l-engine cars with 220 km/h marking in the speedometer [a speed they usually don't achieve even in freefall :-)] ?? ) -- in my GM Celta, the sweet spot is at 100 km/h [~60mph], so speeds lower than 100 km/h are usually reading HIGHER than real and speeds higher than 100km/h are usually reading LOWER than real. The speed limit in our highways is 110km/h.

DISCLAIMER: I was a software developer for a road engineering company for one and a half year.

Sorry but you are wrong

GPS units compute your speed by computing the difference between your current position and your previous position divided by the time between samples. There's no other way to do it. Doppler is not involved.

The time between samples is what's important here. If it's only a few seconds then there's a good case for innocence. If on the other hand it's 30 seconds or a minute, the cop with the radar gun wins. BTW, it is the radar gun that uses doppler to measure speed.

--
This space for rent

Punk kid disputes ticket, news at 11!

He has somewhere between 0 and no chance to win this. Who gives a shit what his GPS says. If the radar gun was properly calibrated and can be documented as such, it makes 0 difference - he's screwed.

What's to prevent me from doctoring the GPS log?

On my system the GPS application stores its logs in a textfile which I can easily edit. It would be trivial for me to doctor the text file to contest any speeding ticket. I'm not sure that this is a good form of evidence.

GPS more accurate than radar?

I've been sitting in my car PARKED waiting for people and seen my GPS speed go up to 5 mph... I don't see how this can possibly more accurate than a radar gun. a margin error of +/-5 mph seems pretty crappy. (it's a newer Garmin, less than 4 months old.)

I have used this

I used the GPS defense when pulled over.

In San Antonio, TX I was pulled over for doing 76 in a 75 zone. I successfully argued that the GPS was more accurate than the RADAR, when I said that it used "government satellite signals."

In fact, most police radar units are +/- 3mph. A consumer GPS speed indicator is typically accurate to within