In Cecil’s column “How Do Radar Guns Work”, Cecil explains how the radar gun works, but… I want to know how does a cop know s/he’s hitting which vehicle in busy traffic??? Is the cold truth that s/he doesn’t?!? :eek:
With radar it’s part of the training to know, but, yeah, sometimes it’s not easy. The car in front is not always the car that is on the radar display. A Corvette could be going 60mph and a large semi-truck could be coming up behind it at 75. The truck will cast a larger shadow and the unit may lock onto it. An inept officer might mistakenly credit the Vette for the 75 because it was the car in front of the pack. There are multi-vehicle and multi-lane traffic radar units that help combat this. But they are expensive and few agencies use them. But any officer that went through the certification course will be proficient enough not to make errors like this.
lidar (laser) is different. Every Laser unit I’ve used had a scope with a red dot site on it. The dot is aimed at (usually the license plate) and blip. That is the exact car you clocked. BTW, @ 1500 feet the circumference of the dot on a license plate is about the same size as that of a coffee can, give or take. So your Valentine One sitting on the dash or visor probably isn’t going to see it down there and will remain silent. Laser detectors also have a hard time seeing laser being aimed at the vehicles ahead of yours. And all laser guns are “instant on”. So even if your detector does chirp it’s too late. Pull over a lot of folks that have them.
To piggyback (no pun intended) off of what was said, the radar unit is there to verify the officers observations. The first part of speed enforcement is to observe the vehicle going at a high rate of speed then verifying it with the radar unit. I don’t know what goes on in Cheeseland with pkbites but the first part of our radar certification is to go out with an instructor and estimate the speed of various vehicles. With enough practice anyone can be pretty damn accurate. After meeting the requirement for estimating speed you make the move on to operating the device. If you don’t get lazy and just stare mindlessly at the display it is pretty easy to tell which vehicle is going fastest.
Our training is a minimum of a 40 hour course and it’s a state certified course. Laser is an additional 24 hours. Neither is mandatory but it does help in court to have it and I know of no officers that run radar/lidar that haven’t taken it. When Laser first came out in the early 90’s there was no state training and all instruction was done by factory reps. I have never been trained in nor used VASCAR. The only agency around here that uses that is State Patrol.
During my first career we used to use moving units facing backwards spraying behind the squad. Vehicles that were driving up knocking on our back door where getting clocked but the drivers would think they were safe because they didn’t realize they could get clocked that way. It did take a special skill to know which car was causing the reading while driving and observing traffic from behind in a rear view mirror. 21 of my 25 years there were on a highway unit and I never lost a speed case.
My current gig we have 1 handheld K-band (kustom Falcon) and the rest is Laser (LTI 20/20). We have no moving units. I prefer Laser, as does everyone else I know.
I worked with a NASA laser expert who defended many speeders (as an expert witness) when laser was used. He never lost a case. Laser has many problems and the manufacturer (LTI) would simply not show at some point during the trial to avoid setting a precedent in which the accuracy was proven inadequate.
In the case of the light beam’s size, it has nothing to do with that tiny dot the officer sees. The LTI 20-20 (the most popular gun at the time) uses 3 laser diodes side by side, aligned so they merged to give a stronger signal. Each diode’s output beam is about the shape of a football oriented with the long axis vertically. The three beams together looked like this (scroll down a bit):
Note, that is not my friend’s web site. At maximum approved range, they cover the entire front surface of the target vehicle. It is quite plausible to hit the license plate, the headlights, the big truck bumper next to the target, etc.
The LTI 20-20 does not take instant readings. It takes something like 25 separate distance readings over a prescribed time period and plots the distances as a line, then takes the slope of the line (distance vs time) to establish speed.
The unit had many problems that the company was not willing to discuss in court, nor divulge any algorithms. To begin with, their clock was simply not fast enough to measure distance at their stated accuracy. A error of 1 foot in the calibration distance the officer uses produced a fairly large error.
When plotting the line, an algorithm threw out “bad” data points. LTI would not reveal what data the unit kept. Nor would it discuss the dual slopes if the beam first measure one target, then switched to a stronger, nearby target (a truck bumper several feet ahead of the target vehicle). Did it use the first slope? The second? Did it average the two slopes? That’s why they always vacated the case.
In my friend’s most famous case, he had the defense lawyer pick up the laser gun and point it at the adjacent courtroom wall and trigger it while just barely turning the unit. He clocked the courtroom wall at 100 mph and showed it to the jury.
Another way drivers won was if the target distance was too far. The 20-20 was only authorized to 800 feet, after that the large beam size and weak response could not properly establish speed. That distance must be stated on the ticket and if it was over 880 feet, they could not use the evidence.
Paul spent many an afternoon scribbling equations on my blackboard as he discussed a case’s outcome. He, by the way, was on the team that designed the distancing laser that shuttles use to dock with the ISS. I have been retired for 12 years, so things may have changed since I last talked to him.
Never piss of a scientist that is a car enthusiast.
Dennis
That column was written by Q.E.D., not Cecil Whatsisname.
**Q.E.D. **was more of an electronics fundamental geek. So “real life” issues would be unlikely to be covered.
Yeah, speeding trials rarely get that technical or intricate. I’ve never known any rep from any manufacturer to testify at any of them. But honestly, only about 5% (if that) ever go to trial.
I first went on the job back in '82, just a few years after the infamous “Florida speeding tree” incident. Attorneys trotted out all kinds of things to fight radar tickets. Then as now the conviction rate for radar was still over 95%.
Nowhere on our cites is there an area to show the distance for a laser reading. Though the lidar units are capable of displaying that information. About 99% of my cites are at 500-1000 feet. But I have gotten good readings at 3/10ths of a mile and nothing around here says I couldn’t use it. Some of the older laser guns are a pain in the ass because they can’t be used through a windshield. Which means a squad has to be parked in such a way that a window can be down to acquire a target vehicle.
Some years ago a co-worker was given a radar ticket for doing 35 in a 25-mph zone – on his bicycle. He simply showed up in court and said, “Your honor, if I could peddle my bike at 35 miles an hour, I’d be in the Olympics!” Case dismissed. He surmised the officer picked up a vehicle behind him.