Speeding tickets

[nloding]

This is for people who either a) have an inside track of how police equipment works; or b) have an engineering/physics/whatever background and understand what's possible.

I understand how radar speed guns work -- the doppler effect, got it. I understand how laser speed guns work, and why they are incredibly accurate. I got it. Here's my question:

If a cop is driving toward you, can he 'accurately' tag your speed??

I was recently pulled over by a cop who was driving towards me. I came over a hill, going 60mph-ish. He made u-turn behind me and pulled me over. He asked me how fast I was going and I said "Sixty." He said I was going 67 (I might have been, coming over the top of the hill, but I was on my brakes slowing back down). Besides, it was apparently a 55mph zone. He wrote me a ticket for 5 over.

Anyhow, can he get my speed when he's driving towards me?

My science/match background says "No way." You have to take into account direction, the vector of the oncoming object, to get it's speed accurately. You can just say "I'm going 50mph, and it's closing in on my at Xmph, therefore it's going this fast." Because if I was angled away from him at a 45 degree angle, that speed would be different. Yeah my car would be closer to him, but it would reflect a slower speed (if I did my math right). Either way, it's still a different speed depending on the angles of the cars.

I'm tempted to challenge the ticket. Can he tag me like that? I did some Google searching but only found information on how radar/laser speed guns work ...

 

[nintendofreak]

TGFPC!!
(thank god for physics class)

Those things work relative to an axis of points, which is the gun/device itself. Assuming the speeds were constant for at least 1 second after he passed you, you could think of it this way...


At the moment you meet, we'll call the origin

C=cop
O=origin
Y=You

------------------COY-----------------------

after that, you can measure the speed of the cop and you relative to the origin (distance=velocity*time , Velocity = distance/time)

-------C----distance-----0-----distance----Y-------

These values for velocity are constant assuming you were going the same velocity before you met at the point of origin... So he would have to get the radar gun reading, and subtract his velocity from the reading on the radar gun... But who the hell drives 55 mph holding a radar gun while going up a hill?!?

With a traditional radar gun, he shouldnt be able to determine that, because if your going 50, and hes going 50, and you are going in opposite directions, the radar gun would register 100 since he is also moving. This would have to mean that if he was able to track your speed, it would have to be with some computer software, which takes a traditional reading, and then subtracts his velocity.

Did he/she tell you how the speed was measured? Hes probably BSing. Those things are pretty accurate, if you said you were going 60 (in reality 67) and he gave you the ticket for 5 over, then its obvious that he messed up with measuring speed. If hes "accurate" enough to measure a 5 mph speed over the limit, im pretty sure he would measure a 17 mph over the limit rating, 3x more than he gave you for the ticket...

 

[kellyan95]

Bring in a physics book, and throw it at the cop. (Pun on the phrase, "The judge threw the book at me")

 

[nintendofreak]

For those too lazy to read, just read this

:

Did he/she tell you how the speed was measured? Hes probably BSing. Those things are pretty accurate, if you said you were going 60 (in reality 67) and he gave you the ticket for 5 over, then its obvious that he messed up with measuring speed. If hes "accurate" enough to measure a 5 mph speed over the limit, im pretty sure he would measure a 17 mph over the limit rating, 3x more than he gave you for the ticket...

 

[MADKATZ99]

5 over? Pffft, you pussy.

That wouldnt of happend if you were doing 100 over

They would look at it go by, and think... "screw it".

 

[FAST6191]

While there is a lot of shady forensics (search for ear printing, there was a rape case about 5 years ago relying on that overturned by DNA) speed stuff is not with that.

If it is just a fine I would say pay and walk away (speed normally means points on license around here which in turn means hiked insurance for some providers).
There are also several things you can do to frustrate efforts of speed devices although I have a feeling the law regarding their use varies by state.