Radar/Laser guns

[Dash8Driver]

I would just like to put to rest some "Urban" Myths I've heard about speed measure tools the police use.

Is it true they are not supposed to use the "Gun" when you are coming towards them, while the cop is driving towards you, ie head on. The reason behind it being the relative velocity. If Im doing 70 and the cop is doing 50 the gun would show a speed on 120, the same rule applies when goin away. So the only way to show your exact speed would be for the cop to manually adjust, ie subtract his speed at the time of clocking from the speed measured or if the gun is somehow tied to the cars computer automatically adjusting for the speed. Either case presents the problem of how do you know the calibration was correct on the cop's car?

Second question. Does the cop have to show you the readout showing the speed and the calibration results for the gun for that day?

Again these are just the rumors I have heard through the years and I'm just curious to accuracy or inaccuracy of them.

Thanks

D8D

 

[James Young]

Dash8Driver writes:


Is it true they are not supposed to use the "Gun" when you are coming towards them, while the cop is driving towards you, ie head on.

No.

The reason behind it being the relative velocity. If Im doing 70 and the cop is doing 50 the gun would show a speed on 120, the same rule applies when goin away. So the only way to show your exact speed would be for the cop to manually adjust, ie subtract his speed at the time of clocking from the speed measured or if the gun is somehow tied to the cars computer automatically adjusting for the speed. Either case presents the problem of how do you know the calibration was correct on the cop's car?

Most of the modern radar units use two different beams in moving mode, one to ascertain the velocity of your vehicle relative to the patrol vehicle (PV) and another to ascertain the velocity of the patrol vehicle by bouncing it off of the roadway ahead. No manual adjustments are required. It can create an issue with cosine error if this second beam happens to measure the speed of the PV relative to a sign or another car off to the side rather the roadway straight ahead. That would understate the PV speed and therefore overstate your speed. Different states have different requirements as to calibrating radar units, some requiring at beginning and end of shift, while others require before and after each stop. This is calibration with the tuning forks, not the internal calibration that some cops will try to slide past you. The forks themselves also must be calibrated periodically, usually every six months.

Second question. Does the cop have to show you the readout showing the speed and the calibration results for the gun for that day?

Generally, no. But all that readout proves is that it shows a particular speed. You have no idea where it came from and neither does the cop, although they’re supposed to be trained to visually estimate speeds and then verify with the radar. Now, we all know it isn’t done that way in the field. Another issue with looking at the radar is that the display may have come from a small aircraft landing at the local general aviation airport, as was the case in Florida a few years ago. Another drawback to asking to see the unit is that it gives the cop just one more thing to remember you by and he will note it on his copy of the citation. If you really want to know what kind of unit it is (very important information), note the number of the PV and go by their HQ some day and look inside that PV.


Go to http://www.caranddriver.com/idealbb/forum.asp?forumID=3. Lots of good stuff there.

 

[CdwJava]

If you really want to know what kind of unit it is (very important information), note the number of the PV and go by their HQ some day and look inside that PV.

Better to get that in discovery ... if it's a handheld unit, it may be one of several. And if it's mounted in the car, very few departments are going to have their patrol vehicles open to public perusal and inspection - such open availability to the patrol units makes for some very dicey safety and evidenciary concerns.

- Carl

 

[James Young]

CdwJava writes:


Better to get that in discovery

No, the idea is to get the information independent of discovery so that when it is offered in discovery you can compare the two. If they are different, alarms should go off.


... if it's a handheld unit, it may be one of several.

This is true and since lidar is increasing in popularity, we have many more handheld units. The type of electronic tracking device is usually noted on the citation, sometimes with the S/N.

And if it's mounted in the car, very few departments are going to have their patrol vehicles open to public perusal and inspection - such open availability to the patrol units makes for some very dicey safety and evidenciary concerns.

I didn’t mean to imply that Dash8Driver actually enter or even touch the PV. When the PV is parked in a public area, one can easily see what make and model the radar unit is by looking through the window, creating no safety or evidentiary concerns.

 

[CdwJava]

No, the idea is to get the information independent of discovery so that when it is offered in discovery you can compare the two. If they are different, alarms should go off.

That's fine. But you may not be able to GET that information outside of discovery. In an agency my size, it would be little problem to come across one of our cars in a public place where you could glance in the window ... though you would only be able to get the make off the unit and MAYBE the model - certainly not the serial number. And even in agency the size of mine, finding the particular car would still be an act of incredible luck.

But in a larger agency, good luck ever coming across that car again.

So, it may be one option, but counting on finding that particular vehicle again is a little bit of a longshot. And if it was a handheld unit, then it won't do much good to look in the car anyway.

- Carl