If there is a truck pulling a boat 2 car lengths in front of me, can radar from opposite direction be wrong?

Question

There was a truck pulling a boat about 2-3 car lengths in front of me and a motorcycle cop coming from the opp. direction said he clocked me at about 15 MPH faster than what I saw when I looked down. I do speed on occasion, but not that day! Is it possible there was a shadowing affect?

Answer 1

"Shadowing affects"...hmmmm   Never heard of it.

Answer 2

Yes, it is possible.

Since I was not there, and do not know the exact circumstances I cannot say how likely it was, but it is possible.

"Shadowing" is not the correct term.

Radar units send out a signal and receive it back after it is reflected by other objects. The unit will interpret the strongest signal as the background or ground reflection. This is what reflects back from all stationary objects like the road, trees, houses, signs, etc. and is the speed of the officer's vehicle.

It will read the next strongest signal as its target. The target speed is determined by subtracting the ground speed from the closing speed.

If I am approaching the target at 130mph (closing speed), and I am doing 50mph (ground speed), the target is doing 80mph.

"Shadowing" is an error caused by my following a large vehicle closely enough for the radar to interpret the reflection from that vehicle to be the ground signal. This is how that error works - my speed 50mph, large vehicle in front of me 45 mph. Radar interpretes that to mean that my speed is 5mph. Closing speed 130mph adds up to target speed of 125mph when it is only doing 80mph.

There are several other situations that can cause a radar unit to display the wrong speed. One of the most common is that the radar unit is not "seeing" the same vehicle that I am.

For many years there was a beleif that the radar unit would display only the speed of the largest vehicle. I can remember motorcycle magazines telling their readers that it was impossible for radar to pick up motorcycles when other (larger) vehicles were in the area. This was never true. I have frequently watched a single large vehicle come down a long straight road toward me at a steady speed of 40-50mph. Suddenly, with no change in the target's speed, the radar display shows a solid 90mph reading. A few seconds later the motorcycle that it is tracking comes into my view, flies around the truck, and the display once again shows the truck's true speed.

This is why the rules of evidence require us to visually identify the speeding vehicle first and use the radar only to confirm our own observations.

Answer 3

No such effect. The thing about radar is that it is considered only to be supporting evidence.It is the observation of the officer in that he saw the vehicles on the road and yours was the only one in the beam that was capable of delivering the resultant speed registered on the device. Have a look on google or wikipeadia about doeppler effect - this is operating principle of speed radar devices