Recently a high shool friend of mine got nailed with a $450 speeding ticket on a back country road. He'll also have to get pay for the class to get 3 (not sure, might be 4) which is 4 hours on two Saturdays. Overall, with insurance price increasing, his little ticket will probably cost around $1,200 total. Now when he got the ticket the cop was going a fair speed (40ish) in the other direction (2 lane road). Physically the rate of change of speeds between them is greatly increased as the cop was going the other direction. My friend believes he was going less than the cop said, and his younger brother was watching he speedometer and said the same. Are the circumstances of him being pulled grounds to fight the ticket (with a decent chance of winning)? Can regular radar guns that a cop might use account for the difference in the rate of chamge of speed? The cop said he was going 32 over and that just sounds plain wrong. Thanks in advance.

  • Of course, the scenario you describe involves someone who has already been convicted and sentenced, either due to a plea bargain, or at a trial. And, either a judge or a jury, if there was a trial, found beyond a reasonable doubt that he was speeding 32 mph over the speed limit, a finding of fact very unlikely to be disturbed on appeal even if a reasonable judge or jury could have reached a different factual conclusion. So, he's stuck even if he could have had options at some point to avoid the conviction. He can't fight the ticket now.
    – ohwilleke
    Commented Aug 8, 2017 at 1:38
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    I am not an expert on radar guns that police officers use, but if the gun knows the rate of speed of the vehicle it is traveling in, then some simple mathematics could be used to figure out the correct speed that the other car is going.
    – mikeazo
    Commented Aug 10, 2017 at 16:30
  • There's several ways the radar gun, or whatever display it is feeding, or even the officer themselves, could determine your friend's actual speed regardless of whether or not the officer is moving. (1) Gun reading minus cruiser's speedometer reading. (2) Gun reading minus GPS-based speed reading for the cruiser. (3) Gun reading minus additional radar reading that monitors the cruiser's speed. Any of these could be implemented technologically (using a computer attached to both sensors, or the gun itself if adequately equipped), or manually if the officer has both values in view while driving.
    – Iszi
    Commented Sep 19, 2017 at 19:55
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    Regarding the accuracy issue that @user6726 mentioned: This is a real thing, but it works in favor of the driver - i.e.: it results in a SLOWER reading. So, even if a cop is busting you based on a reading that's skewed by observation angle, the inaccuracy caused by that angle is NOT something you want to use as a defense.
    – Iszi
    Commented Sep 19, 2017 at 19:57
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    For me, the one crucial detail that is missing is what type of gun was used - was this the hand-held kind, or a properly configured vehicle based one. The latter knows how fast the vehicle it is mounted in is going, so can do the maths itself to derive an actual speed from the closing speed, with "reasonable accuracy". Hand-held guns assume that they are stationary, and are completely incapable of giving a correct reading from inside a moving vehicle.
    – MikeB
    Commented Feb 12, 2020 at 15:41

1 Answer 1


Moving radar gun readings are held to be admissible evidence, if used properly, and are certified to be accurate within 1 MPH. Reportedly, they are not accurate if the angle is more than 11 degrees (or something like that); in addition, there are distance limits (too far and the reading is prone to error); and it needs to be calibrated. Police training records can be subpoenaed to argue that the officer was not properly trained.

Low-tech speed determinations are admissible, provided the officer is following at a consistent distance (obviously going the same direction). Another is the actual time taken to pass two highway marks. This could be unreliable when the officer's vehicle is moving towards the suspect's vehicle (one's perception of when a person has "passed" a distant point depends on your angle relative to the fixed point, and that angle is changing).

In the case of US v. Sowards, the defendant was pulled over for allegedly speeding (and drugs were then discovered), so the question was whether there was probable cause. In this case, the arresting officer "just knew, from experience" that the car was going 75 in a 70. The opinion includes the officer's testimony, which really should be read. The district court upheld the arrest, saying

He’s trained to estimate speeds. His difficulty with measurements is immaterial to his estimate of speed as that did not depend on time or distance

However, the 11th Circuit Court (which overruled the district court) found contrarily that the officer was not "trained to estimate speed", he was allowed to guess, and trained to use radar (he did not use radar). This does not mean that visual estimations are inadmissible, but they are vulnerable. Thus there is a legal industry of fighting speeding tickets, and people who expertly know the scientific ground on which the evidence could be challenged.

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    If the cop and my friend are moving away from each other then the rate of change of the distance between them (assuming it's straight line) is the sum of their two speeds right? So how could a radar gun possible account for that drastic difference (65+40=105) when it has no way to calculate its own speed as opposed to if they were moving in the same direction (65-40=25)? The speed is changing 4 times faster because they were going in opposite directions. Even worse-I drive that road everyday-he was on a turn so the outside side of his car (clocked) was going faster than his average speed. Commented Aug 10, 2017 at 13:06
  • I don't know the details of the technology, but it involves two antennae. It does have to be a moving radar gun.
    – user6726
    Commented Aug 10, 2017 at 13:41
  • @demcooltricks the speed isn't changing unless one or both are accelerating/braking. Also speed differential between left and right side of a car would be minimal until you were at a low enough speed to be able to turn at a sharp enough angle for it to be significantly different, at which point you wouldn't be speeding anyway.
    – Smock
    Commented Feb 13, 2020 at 12:11

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