Mantua - UT Half Mile Laser Hit & Question

[RRJ]

INFO-Be aware that Mantua is in fact monitoring south bound traffic more, so be careful in either direction.

QUESTION: Today I was traveling south bound in Mantua and got a laser hit in my SUV. This is extremely rare as I have my R3 mounted up high in the tint strip, far from my headlights and front plate. The LEO was about a half mile away shooting what seemed like too much of an angle to be effective. When looking at the map the angle doesn’t look to be as wide as it feels in person. I’m trying to find info on what angles LIDAR is effective. Is there info here or somewhere else I’m missing?

I’ve attached crude photos with my best finger drawings of the situation. The blue line is the angle of travel. The red line is angle the laser was shooting. The intersection of the two is about where I was hit. The red X that looks like a toddler made is where the LEO was sitting. The other red marks away from the highway are accidental and can be ignored. Like I say, the angle feels stronger in person but maybe it makes little difference anyway? Where would I find info on this?

On a side note, today Waze had a much longer range than the LIDAR or R3, so even if I had been speeding, which is rare in the SUV and even more rare near Mantua, I had ample warning.

 

[sdrawkcaB]

LIDAR (laser) and RADAR can both be used slightly off axis. The greater the angle, the more inaccurate the reading goes down. This is called the “cosine effect”. Radar is much more prone to the cosine effect than laser, and this often has to do with LIDAR guns software tracking capabilities; Laser guns are much better at telling when measurements are abnormal, and varying too much in the moment. If the measurement is too abnormal as it is being taken, the laser gun will filter out the readings and not produce a reading. Though, they are not perfect.

I can actually use my LTI LR B LIDAR gun and stand still, and “sweep” the gun across a large flat surface such as the side of a large building, and produce a false speed reading. While the building nor I are not moving, I can make the gun believe it is measuring a moving object purely by taking a speed measurement while I sweep across the surface of the building, going from a point on the building that is closer to where I am standing, to a point on the building further away from where I am standing.

This page does a great job describing the overall principle of Cosine Effect, though I disagree with the overall final pages opinion on if speed can be measured off-axis. In my opinion and experience, yes, speed can be measured off-axis. Will the measurement be 100% accurate? No. Should it be measured off axis? No. Can it be measures off axis? Yes. Will it be measured off axis? Apparently from both your and my experiences over the years there through Mantua, yes.

Cosine effect will always result in a lower-than-actual speed reading of the targeted vehicle. For a vehicle approaching the LIDAR gun in a curve, the reading will become more accurate as the vehicle continues to approach the officer. If the officer is at the start of a curve, and measuring you as you travel away and around a curve, then the measures speed will become increasingly inaccurate.

All of that said, LIDAR can still produce a reading for up to ~20 to 30 degrees off-axis, depending on the make and model. After 30 degrees, most guns won’t give a reading. Radar on the other hand, can frequently still produce readings, despite being extremely inaccurate.

The biggest takeaway from all of this, is to remember that cosine effect always works in the drivers favor. If you got stopped and questioned about a speed, you wouldn’t have much sympathy from an officer when you try to tell them that the reading they got of you speeding, was inaccurate, and that you were actually going much faster than what they measured you as (because again, the cosine effect always results in the measured speed being lower than the actual velocity of the measured target).

 

[Marty K]

Very informative, @sdrawkcaB.

@RRJ, at that distance (half mile), the lidar beam would be over 6 feet wide for most guns, so your RD detecting it actually is not that surprising...

 

[RRJ]

Good to know.

Post automatically merged: Jan 21, 2020

I can actually use my LTI LR B LIDAR gun and stand still, and “sweep” the gun across a large flat surface such as the side of a large building, and produce a false speed reading. While the building nor I are not moving, I can make the gun believe it is measuring a moving object purely by taking a speed measurement while I sweep across the surface of the building, going from a point on the building that is closer to where I am standing, to a point on the building further away from where I am standing.

If a sweeping movement can cause error, what about normal human hand movements at 1/2 mile? Hand movements are amplified the further out the beam goes so it would seem a movement with little impact at 500 feet could have a larger impact at 2500 feet, supposing that’s an issue.

 

[sdrawkcaB]

You're correct, hand jitter can cause issues. Officers compensate for this by parking their vehicles perpendicular to the road way, rolling down their window, and then stabilizing the gun against the bottom of the window opening on their car door. Higher end newer LIDAR systems can actually shoot through the windshield, and officers will sit in their cars, parked parallel to traffic, and shoot vehicles passing by, stabilizing the LIDAR gun against their steering wheel.

It is much harder to get distance readings at a greater distance, but definitely doable. If you're a traffic cop and you do this all day, every day, then you've got plenty of time to practice and become proficient at it. In Europe, it's very common to see an officer mount his LIDAR gun on a camera tripod, to stabilize it while shooting. They'll then radio in which vehicle to have another officer pull over while the LIDAR operator continues shooting vehicles.

Most of the time LIDAR is used at distances of ~1000 feet, give or take, but there are still members here who get shot regularly at distances of 2000+. One member here got shot at a distance of 2700+ feet by an officer this past year. In Provo, UT, there is an officer who shoots people on a 15mph road, at distances as short as 100 feet. If you were to graph out the average distance of LIDAR shots, You can think of the average shots around 1000 feet as the center of the "bell curve", but there are absolutely still outliers on both ends of the average distribution; as little as 100 feet or 2700+ feet or greater.