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The Lowdown on Lidar

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Re: The Lowdown on Lidar

#221

Post by Trombone5 »

darkmighty wrote: post #169

I think he missed a couple important points. The sweep effect, shaking and aiming are more amenable to signal processing than it seems, depending on the resolution and noise in the system. I tried to illustrate in pictures: 1) Just to demonstrate that a nonuniform sweep is amenable to detection or processing, consider this picture, with the correct estimate in green: http://imgur.com/FsPAwCn 2) Now imagine the device…

I think its funny how he claims location sampling at different times is not a measurement of speed.

It's strange how he mentions the sampling speed, but then pretends it's 1/3 second. Basically what he is attacking is the worst design of a speed gun that he could imagine from the public information. A more fair challenge would be to find sufficient error from a competent design of a speed gun, but then it wouldn't be as self serving.

Re: The Lowdown on Lidar

#222

Post by jervisfm »

dtrizzle wrote: post #196
jstalin wrote: post #96

In Michigan, a speeding ticket is a civil infraction, not a criminal offense, so the legal standard is preponderance of the evidence , not beyond a reasonable doubt , as the slide deck says. I suspect it's the same in California.

In CA, infractions are crimes so the reasonable doubt standard applies. See Cal. Pen. Code section 16. http://law.onecle.com/california/penal/16.html

Wait a minute, if that's true won't it imply that traffic violations/infractions convictions get you a criminal record ?

Re: The Lowdown on Lidar

#223

Post by john_b »

Shivetya wrote: post #209

How does the officer confirm which vehicle the LIDAR selected? I have never been clear on this point.

Having fought a LIDAR ticket before, the officer simply claims that he or she targeted your car and the judge admits the claim as a fact without question. Any attempts on your part to question this will be met with a reminder that the officer is Certified and Trained and you are just a random person with no qualifications to question the officer's abilities.

You can ask the officer if there was a wind that day, and if so how heavy. You can ask if the officer used a tripod or held the gun freehanded. Even if the officer admits that there was a heavy wind, the gun was used freehanded, and he or she pinged you in traffic at 950 ft, the judge will still accept the officer's word that your car, and only your car, was targeted.

Why? Because it's easier for the court, because they know you can't prove without a doubt that the officer targeted a different car, and because it is an important source of income for the city or county which has summoned you.

It's a scam, but it's one which Americans in general are comfortable with because it gives them the illusion of safety.

Re: The Lowdown on Lidar

#224

Post by gwern »

puredemo wrote: post #219
shangxiao wrote: post #131

Earlier quoted context omitted.

It was the fact that the guy was Joshua Bloch which got me to read the presentation - and I'm totally glad I did! I would never have read it if it was just named "The Lowdown on Lidar"

So appeals to authority work really well on you..

BTW, your downvotes are an opportunity to reflect on why your comment was obviously wrong and counterproductive.

Re: The Lowdown on Lidar

#225

Post by onan_barbarian »

legohead wrote: post #67
ihaveajob wrote: post #12

Speed radars are good for speed traps, but not so great to prevent speed related incidents. They share the same fatal flaw with all other single-point speed meters: You can just slow down for a minute, while you pass by the speed trap, and then speed up for the rest of your trip. Instead, a 2-point speed meter (i.e. license plate readers every few miles, measuring time between matching reads) system is superior becau…

If you've traveled around the country, you've invariably come across those "speed is monitored by aircraft" signs, and what you described is how they do it (tracking how long it takes cars to go from point A to B)

I've always been a bit nervous about those "speed is enforced by aircraft" signs (as opposed to "monitored"). I always get a mental picture of a aircraft strafing near my car until I slow down...

Re: The Lowdown on Lidar

#227

Post by BrandonMarc »

bliggity wrote: post #194
Arjuna wrote: post #150

Earlier quoted context omitted.

"I'd really like to see more transparency when it comes to title changes." I think dang is working hard to be transparent about this topic. You can see a number of examples of this by reading through the following search result [1]. Cases in point: He has reverted titles to their original form... "Ok, we'll change the title back again." [2] He is open to suggestions... "If anyone suggests a better title, I'll change…

Title changes are a perennial point of friction. Much time and energy by many parties has been spent over months debating and analyzing it. Why not just implement a title history feature? Mods can change title, then users can click a link to show title history. For extra points, you could make the title history page a poll/voting system which allows users above n karma, registered for x months to vote on their prefer…

I definitely like the option of a title history + voting on which you believe is a "better" title. I also agree that limiting it to a certain threshold of karma, account age, etc, is appropriate.

Re: The Lowdown on Lidar

#228

Post by Relaxx »

I'm surprised the angle of measurement never came up. Isn't it the case that a reading has to be head-on for it to be accurate? It seems pretty obvious that at higher angles the observed distance delta will be smaller.

Re: The Lowdown on Lidar

#229

Post by lawnchair_larry »

adamtj wrote: post #205
kudu wrote: post #83

Why do HN mods keep changing the title from meaningful ones, which make the reader interested in the content, to generic, vague ones? You'll see that there's definitely a correlation between the moment that HN mods change the title and the post dropping off the top frontpage spot.

I would assume such changes come from a desire to be accurate. But accurate how? In what way? That depends on the purpose of the title. As I see it, our differences of opinion on this topic stem largely from misunderstanding it. I think it's not at all clear what we're even talking about, so we wind up arguing about slightly different things and fail to come to a consensus. I think that we need to really understand t…

It's extremely confusing to me how the mods still can't comprehend this.

Re: The Lowdown on Lidar

#230

Post by disillusioned »

I have lidar jammers on my car, but Arizona doesn't use a lot of lidar. (Motors and sometimes bored DPS will, but typically it's Ka band radar via Stalker Duals.) Not really knowing how lidar worked, except that a "laser" pulse hits the car and reads back a speed, this article was great because it explains that the measurement process is far slower than you'd think (0.3 seconds!) which is easily enough time for my jammer to detect the inbound infrared and start spamming light on that frequency, which is what they do.

I don't think I've ever successfully jammed one, since again, AZ is mostly Ka, but it's pretty neat to know that they can work as advertised (since they're built in to the front of the car most likely to be targeted by lidar).

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