I was traveling down the road yesterday on the way to work on the motorcycle and I passed a cop who was positioned quite a distance off the road. I noticed that he was aiming his radar gun at me and following me with it. I was only going about 5 over the limit so nothing happened.
Is it possible to get an accurate speed this way?
He was about 25-30 yards back from the road, or about 75-90 feet from my best guess.
How could a radar give an accurate reading unless the distance from the road was known and/or the angle of the initial reading and the angle of the last reading were known?
Seems to me that if the radar gun was aimed at an angle to your direction of motion it would read low, not high. So it would be to your advantage if the angle wasn’t corrected for (consider the extreme - if he was foolish enough to aim 90 degrees to your direction of motion, i.e. directly to one side, the radar gun would say you weren’t moving). As to whether in fact the angle is corrected for, I don’t know. If the cop was very consistent with his aiming it could easily be corrected for mathematically, but I have my doubts.
My understanding is that police radar functions by taking two consecutive readings on a target (a programmed-in double check). I find it hard to believe that there is an angle/vector correction built into the unit, particularly considering that it would have to be referenced to the line of the street (how would one do that?) and would have to calculate for two different angles with the moving vehicle. The plain old radar function is routinely calibrated, and that combined with the instrument’s readout is pretty well accepted as firm evidence in court. But figuring a vector would require additional info (angles and distance) that are not measured by the gun, but would have to be provided by the user. This would leave lots of room for human error and I think would easily be challenged.
My best guess is that the officer in question did not really understand the limitations of what he was doing, and that he probably didn’t get any readings over the speed limit.
And yes, as Canadjun mentioned the uncorrected reading would be low. Can you imagine the officer trying to explain to the judge “Well, yeah, the figure was 5 mph below the limit, but according to my calculations that means he was going 10 over the limit.”? Not going to fly, I’d say.
Yeah, I did figure that the readings would be lower than normal straight-on shots, however, if he paced off the distance and then took two readings, say, exactly 3 seconds apart would he be able to come up with a reading that would “stick” in court?
That would also assume that I haven’t accelerated or deccelerated during the time he takes the two measurements. I can’t see any scenario where these readings could be considered accurate enough to pull someone over, issue a ticket, and have it stick.
I have serious doubts. Presenting authoritative understandable testimony based on verifiable (to some degree) accurate distance and angle measurements and the necessary computations, which may be hard to follow, would be a tough job. Far easier to find a hiding spot closer to the road.
Yes it will stick. Why take two measurements? As long as it can be proven that the radar unit was not functioning properly. It is well established in court that any reading taken at an angle gives a reading that favors the driver. You might be going 15 mph too high and the reading says 10. He can’t try and calculate the speed, you’ll get a ticket for 10mph over.
If you mean what was he thinking by being that far off the road, maybe he wasn’t trained properly about what can effect an accurate reading. Probably not, but possible. He may be thinking that it was a good safe spot to run radar and he’ll get the speeders the speeders going very fast even if his reading might be a little lower than how fast they are really going. Maybe he didn’t have a choice. Many of our radar details are mandated from up on high. Some citizen complains to the mayor about speeders on their road and we are told to go enforce it, even on roads we know that speeding is not a problem. He might have been told to be in that exact place because that is where the complaint came from
If you mean what was he doing tracking the car all the way to 90 degrees? He was probably just doing it for effect. He gets a car that is over the speed limit but not high enough to pull over. He makes a big show of tracking it so the driver slows down even though he has no intention of pulling him over. If he got a reading high enough to pull someone over then he wouldn’t have time to track the car that far, nor would he need to. He would be busy putting the radar gun down safely, placing the car in gear, checking if it was safe to pull out on to the road and keeping tabs on the car he is going after.
Thanks for dropping by Loach, much appreciated.
This happened to be a motorcycle cop, and he did track me to 90 degrees. I figured I was safe because there were a LOT of cars behind me and he would have had to pack up, get back onto the road and make his way through a couple dozen cars on a tight street to catch up to me. It did cross my mind that he was “showing” me that he thought I might be going a bit fast with his act.
Thanks for the info.
A majority of the signal goes out the front, but it’s not all in a straight line. The radar receiver can’t tell if the echo came from dead ahead, or if it came from 5 deg to the left. The radar receiver also can’t tell if the echo came from one of the sidelobes. All the radar receiver hears is the echo from the radio wave that was sent out.
Radars used in fighter jets and such send out a much narrower beam, and the antenna isn’t fixed. It moves around tracking through the sky, so that the radar can get all sorts of information about position, speed, and can track the object to figure out vectors and such. Police radar is much simpler, and has a single fixed antenna which can’t track diddley.
In a typical police radar gun, the beam width is too wide to allow for the officer to go back and pace of the distance to figure out the actual vector or whatever.
You could call the police department as a disinterested citizen and ask why type of radar gun(s) they use. Assuming you get an honest answer, I’m sure the gun manufacturer would have an operating manual that details how to use it properly. So if you found yourself in the same scenario, and get stopped, a trip to court to show the officer was not using the radar gun according to the operating manual would get your ticket thrown out.
A defence might well be to insist that test data and calibration of the radar gun for use on bikes is produced.
You’ll find that its quite unlikely that it is fit for the purpose, I have seen one case of a bike producing a reading of 40mph, whilst the bike was on its side stand motionless at the side of the road.
Slippage along the length of the bike can cause a significant error too, where the cop tracks the bike slightly faster then the machine, because the width of the beam still produces a reading, even when it is quite a way off the beam centre.
http://radar.757.org/LASERweb.html
There are operating procedures laid down that specify exactly how these must be used, most driver do not think to question the evidence, but if they do, they may well find the device has not been used within the correct envelope.