
The Lowdown on Lidar - timdierks
https://docs.google.com/file/d/0B941PmRjYRpnX3ZqLWlQMGdGXzQ/edit
======
taeric
I confess I was expecting to not care for this. I'm not sure how any of it
should be surprising, as it seemed fairly straight forward analysis of the
tools.

That said, this was a ridiculously fun read. More detailed than expected for
some parts, and always informative. I'm glad it did not come across as anti
speed detectors. Really more of a "know the limitations."

~~~
alphonse23
I hope he realizes this, as much as I hope everybody else realizes this, that
the only reason why he got the violation reduced was because he brought his
attorney on board. Anybody who lives in California whose contested a ticket
knows that 9/10 times (if not even more than that) judges side with the
officers -- whether they have a strong case or not -- the State of California
certainly doesn't want it's residents to get any ideas.. if you know what I
mean.

I wonder if his attorney cost more than the actual citation? Most people can't
afford lawyer fees, so cops in California get away with this small stuff all
the time without any fuss, whether they were liable or not. Congrats Joshua
Block -- BTW I loved "Effective Java"!

~~~
blumentopf
I'm surprised they exonerated him, considering his driving history...

Turns out he apparently blew a red light with his Boxster in 2010 and hit
another car:

[http://attitudeofgratitude.typepad.com/attitude_of_gratitude...](http://attitudeofgratitude.typepad.com/attitude_of_gratitude/2011/11/one-
year-later.html)

~~~
gertef
That's a strange story. According to the blogger's version og the incident, he
was struck an injured by someone clearly breaking the law. Why wasn't the
other driver detained by police and arrested?

------
ihaveajob
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 because a) their margin of error is
negligible and b) they measure sustained speed over a long distance rather
than at a single hot spot, thus making roads safer. I can't believe it's not
implanted at least in all interstate freeways.

~~~
growse
Possibly because the cost/benefit just isn't there.

In the UK, the data shows that speeding (excessive speed for the conditions
above the speed limit) is the primary cause in single digit percentage of
accidents. I believe there's data from 1996 and 2007 for this, will dig it out
when I'm not on mobile.

If you want to spend money on accident prevention, there's much better things
to target than speeding.

~~~
saosebastiao
I have yet to get into an accident, but I actively avoid walking across
several streets in Seattle because of a combination of entitlement thinking
(This road is mine because I'm in a car!), and pervasive speeding. I'm not the
only one...I recently found out almost nobody on my side of a local road will
cross the road due the same problem. The road, therefore, has become a barrier
to commerce and has split a community in two.

Preventing accidents isn't the only reason to enforce traffic laws.

~~~
quux
There aren't any traffic lights and crosswalks on this street?

~~~
saosebastiao
There are some crosswalks, 8 blocks away from each other. No lights for at
least a mile. In WA state, every intersection is automatically a legal
crosswalk with pedestrian right of way (unless marked with a "No Pedestrian
Crossing" sign), but the lack of enforcement combined with the high speeds
makes that right of way effectively meaningless.

~~~
e40
I walk a _lot_ in CA. Not long ago I saw a cop not yield to a pregnant woman
pushing a baby in a stroller, in a 4 lane road. She was in the crosswalk,
clearly marked. There was no light or stop sign, but she was in the middle of
the 4 lanes when the cop drove past her in the closest lane. I really couldn't
believe what I was seeing.

When the cops don't even respect the laws, the laws are meaningless.

~~~
kelnos
This doesn't surprise me. I walk a lot too, and I constantly see cops flip
their lights on for the few seconds required to make an illegal left turn, run
a red light, etc., and then turn them right back off.

~~~
coldpie
Yet another consequence of police immunity from prosecution.

------
ylhert
LIDAR sucks and there is no judicial notice on it in California. If you ever
get a LIDAR ticket, do a trial by written declaration (or TBD). Just state
that you dispute the validity and accuracy of the LIDAR reading and they'll
usually throw it out. Remember to ask the officer(after he has given you your
ticket) what model laser/radar detector he was using, and at what distance he
measured you at. Anything over 800/1000 ft(this is really close, and rarely do
they get you this close) will usually automatically be struck down if you
question LIDAR's validity at that distance. That is a case you will win every
time, if you do it right. The court cannot prove it's accuracy at these ranges
and they know it. Also, rule of thumb, all CHP officers have laser only on
their bikes now. source: I used to have a Lancer Evolution

~~~
gknoy
>> Remember to ask the officer(after he has given you your ticket) what model
laser/radar detector he was using, and at what distance he measured you at.

That's an interesting point. Would that be admissible, though? Wouldn't you
saying "He said it was at X feet" would be considered hearsay in court, versus
a cop saying "I took a reading at X/2 feet"?

~~~
ylhert
He/she should normally write it on the ticket(pretty sure they have to, it's
always been there on tickets I've seen), but if you can get him/her to say it
out loud, it's evidence that you can probably use against him/her. Also you
should record your interactions with all officers. My final not a lawyer
advice on beating speeding tickets: shut your mouth and don't answer the
officers questions. If the officer asks how fast you were going, nothing you
can say helps you, especially "I don't know". Just do your best to be polite,
but firm, and don't give him anything more than a nice attitude

~~~
lawnchair_larry
This isn't always the best strategy. If you're unlikely to make a court date,
being remorseful and apologetic has a decent chance of getting you a reduction
from the officer. If you play hardball, so will he, then you have no choice on
going to court if you want a reduction.

If you're in serious trouble, STFU is the correct choice, but it's not
necessarily the most economical for a moving violation.

~~~
aidenn0
In Virginia, all moving violations are arrestable offenses, so I grew up doing
STFU. My understanding is that most states are not that way though.

------
MattGrommes
As a college intern at a national lab my boss' boss got a speeding ticket on
the military base we worked on. He spent some time doing research and when he
presented his evidence in court the judge apparently told him he wasn't going
to have to pay the fine but the judge wasn't going to find him not guilty
because he wasn't going to risk invalidating all speed gun findings. I don't
recall if it was lidar but it's funny the stories are so similar. It's hard to
use technology to mess with engineers. :)

~~~
aroch
Lower courts (even in the Military) can't really establish any meaningful
precedence

------
darkmighty
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](http://imgur.com/FsPAwCn)

2) Now imagine the device is more sophisticated. Consider it has a very large
resolution around the incoming pulse:
[http://imgur.com/z8UrhUw](http://imgur.com/z8UrhUw) In this case the beam
width is actually beneficial, and so are large distances. The sweep effect
would cause a shift in the peak intensity shown in the graph, but the object
depth frustrum should be clear across samples (and dislocating at
t'=t-v*dt/c).

3) Officers can be trained to recognize situations unforable, regardless of
distance. One fast car passing a slow one would make it easy to confuse the
equipment. A clear desert however doesn't present many threats for error.

~~~
Trombone5
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.

------
mrfusion
When I was a teenager I tried to fight a speeding ticket, and prepared a
presentation like this with several good points (I think).

Come court day, I got all dressed up, stood up when called and started my
arguments ... and the judge simply cut me off after two sentences and said
"guilty".

I'm surprised this guy was allowed to present his whole argument and it
actually convinced a judge.

~~~
driverdan
Did you appeal? You can always appeal your first court hearing. The key is to
talk to the prosecutor beforehand. They will almost always plea it down.

~~~
mrfusion
This was in Maryland. It seemed you went right to the courtroom and talked to
a judge along with 40 other people. There was no prosecutor present as far as
I could tell.

I've fought them in Connecticut later and there you only meet with a
prosecutor and they warn you all over the place that if you don't take the
deal and try to go to court the judge can significantly increase the fine :-(

------
justinsb
Can anyone tell me why speeding tickets aren't enforced with video cameras
(like red-light tickets are)? It would seem to be much more accurate, much
easier to enforce, and much harder to dispute.

Irrespective of the rights or wrongs of speed limit laws, I do believe that
consistent and efficient enforcement can only be a good thing.

~~~
dmoo
In Ireland we have a system where you can contest any of these automated fines
in court. The problem is that the fines and penalty points double should you
contest and lose.
[http://en.wikipedia.org/wiki/Penalty_points_in_Ireland](http://en.wikipedia.org/wiki/Penalty_points_in_Ireland)

~~~
kudu
How is that even legal? It seems like it would infringe on the requirement of
due process anywhere in the world.

~~~
koenigdavidmj
That's just a more formalized version of a plea bargain. (Whether plea
bargains are just as much a due process violation is not part of my post.)

------
leephillips
I hope this information, or the equivalent, filters into the awareness of
judges. I was in traffic court in Virginia last year and, as I was waiting for
my case, heard the judge explain to several defendants who were there to
challenge their lidar tickets that the lidar was essentially infallible and
could not be confused by other cars on the road. And this was an intelligent
judge who seemed unusually sympathetic to the accused.

~~~
Guvante
Unfortunately judges are people and if you have tons of people coming in with
just "LIDAR is not accurate enough to prove reasonable doubt" compared to cops
bringing in studies proving their accuracy it is difficult to listen to it.

------
privong
Anecdotally, I heard of an astronomer in a similar situation, who attempted to
prove the radar gun used to "catch" him speeding was incapable of measuring
the speed accurately enough (presenting info on the radar gun's sampling
properties and on sampling theory; I forget the specifics). Supposedly the
judge maintained the speeding violation but reduced the man's fine for
"entertaining the court".

------
zinxq
Josh Bloch is awesome.

For the non-java inclined, he wrote (among other things) java.util.HashMap.
Most all Java programs run (lots) of his code.

~~~
shangxiao
I know him as the author of the awesome "Effective Java"

~~~
thescrewdriver
I know him as the guy who advocated the even wider use of factories...

------
jstalin
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.

~~~
dtrizzle
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](http://law.onecle.com/california/penal/16.html)

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

~~~
dtrizzle
Yes. Infractions are crimes so yes. For the most part, they are not treated as
crimes. You cannot be jailed based on an infraction. Most job applications
I've seen typically explicitly exclude requiring information about past
infraction violations.

------
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).

~~~
tempestn
I've read that in some jurisdictions police have started confiscating jammers
and even charging users with obstruction of justice, on the theory that while
owning them is explicitly legal, using them prevents police from carrying out
their duties. Do you know what the history in AZ is like?

------
DEinspanjer
All the talk in this thread about various forms of speed management and fines
remind me of an article I read a while back about a Swedish test program that
used fines from speeders as the funds for a lottery that people driving at or
under the limit were entered into automatically:
[http://www.wired.com/2010/12/swedish-speed-camera-pays-
drive...](http://www.wired.com/2010/12/swedish-speed-camera-pays-drivers-to-
slow-down/)

------
chillingeffect
The highlights:

1\. Lidar gun manufacturers test them on limited and ideal circumstances

2\. Beam divergence and shakiness make them less useful past 1000 feet, which
is generally acknowledged.

------
PeterWhittaker
Excellent presentation with considerable technical information, e.g.,
weaknesses in the LIDAR approach, operational realities that reduce accuracy,
etc., and some legal background, e.g., limiting LIDAR to 1000' or less in some
jurisdictions - but contrast with overall lack of case law.

Could be of value to anyone wishing to fight a ticket.

------
dzhiurgis
Perhaps anyone can explain me why Lidar is not dangerous to sight?

Whenever you see anyone working with lasers they'd wear protective glasses.
Now I realise Lidar uses infrared wavelength which is not visible to us, but
that does not mean it's not dangerous, or does it?

~~~
bicubic
Non-visible lasers pose a much bigger hazard than visible. Intense visible
light triggers an involuntary defence called the blink reflex whereby we blink
before the retina absorbs enough energy to get damaged. Lasers outside the
visible range don't trigger the blink reflex and can destroy the retina before
you know what happened.

Lidar operates at relatively low power and relies on very sensitive detection
in a very narrow wavelength range to identify reflections.

------
awjr
The reading was taken at a distance of 1300 feet with a handheld device that
is not meant to be used handheld beyond 1000 feet. That's a significant
distance. I'm assuming the cop was hiding or trying to stay hidden.

------
darksim905
A lot of this is common knowledge now. You can check out guys of LIDAR who
research these types of guns & how accurate their findings are. This is also
why cops should be taught to do several speed readings & to point at the
vehicle, not the headlights. Also, depending on the paint, it can also cloak
the vehicle's speed or make it take longer to get back to the gun.

------
Marcus10110
Next time I get a parking ticket, I'll be sure to investigate the temperature
stability of the parking meter's clock.

------
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.

------
voidlogic
I've wondered sometimes if it would be possible to fight erroneous speeding
tickets using the repeated GPS pings your phone or nav unit makes while using
them for directions.

------
ARothfusz
Well, he fought with half science: he came up with a set of testable
hypotheses, but didn't actually run any of the tests. I guess that's as
scientific as cosmology :-)

~~~
thrownaway2424
It's not even half science, it's just a big slide deck of FUD padded with a
healthy amount of entitlement.

~~~
chiph
Be honest with yourself: If you got a ticket where the indicated speed was
massively different than the speed you were actually travelling, you'd fight
it too. And you might do some research into the validity of the reading, as
well.

You'd especially do it in a state where the law says "x" mph over the limit
and your license may get suspended (meaning you walk for 60 days). It'd be
worth fighting for.

~~~
gknoy
Especially if it were in a residential area where you had been deliberately
careful. If I got a ticket saying that I'd been driving 40 in a 25 zone, when
I had been carefully watching my spedometer at ~20s, I'd feel pretty wronged.

I'm not sure I'd have the resources to fight it, but I would certainly want
to.

------
supernova87a
So regardless of the science behind it, what is the successful legal forum /
mechanism to raise doubt about the accuracy of the technology being used by
police?

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

~~~
njharman
I believe they have "sights" and are aimed like a gun.

~~~
icelancer
Cops routinely shoot innocent bystanders.

------
trhway
now if only somebody published a similarly powerful defense on "traffic
control device violation" :)

------
Jimny22
Well the author may be a clever computer scientist but he knows nothing about
physics and LIDAR equipment. Pathetic I'm afraid.

~~~
BHSPitMonkey
Given that (1) you're an expert in these areas and (2) you found no points in
the author's work worth refuting, I guess it follows that the author is
actually an expert as well.

------
kudu
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.

~~~
curtis
I'd really like to see more transparency when it comes to title changes. The
most recent case where one of my submissions had a title change mirrors what
kudu is describing here: a meaningful (and I think accurate) title changed to
a generic and vague one -- followed by a rapid decline in the rate of upvotes.
I think there are legitimate reasons for title changes, and I think that the
Hacker News mods generally do good work. But sometimes they seem to be fixing
stuff that isn't broken. I realize titles in particular can be really
subjective, but that's one reason I'd like to see a comment with the old and
new titles whenever a title change happens. That will allow the community to
chime in when maybe the title change wasn't an improvement after all.

~~~
Arjuna
_" 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 it again." [3]

"That's a fair suggestion and we changed the title accordingly." [4]

"We'd happily change the title if someone can propose a more accurate and
neutral one." [5]

[1]
[https://hn.algolia.com/#!/comment/forever/0/author%3Adang%20...](https://hn.algolia.com/#!/comment/forever/0/author%3Adang%20title%20change)

[2]
[https://news.ycombinator.com/item?id=8050702](https://news.ycombinator.com/item?id=8050702)

[3]
[https://news.ycombinator.com/item?id=7557464](https://news.ycombinator.com/item?id=7557464)

[4]
[https://news.ycombinator.com/item?id=8104621](https://news.ycombinator.com/item?id=8104621)

[5]
[https://news.ycombinator.com/item?id=7959549](https://news.ycombinator.com/item?id=7959549)

~~~
bliggity
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
preferred title. Top voted title after y votes and z time after initial title
change is what's used.

~~~
BrandonMarc
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.

------
ape4
Don't Google's self driving cars use LIDAR.

~~~
diminoten
They probably take all of these things into account.

If they didn't, I suspect Google's self driving cars wouldn't use LIDAR.

------
xchaotic
Exec summary: a geek/nerd gets a speeding ticket driving a porsche. 5 years
later he's still doing powerpoints on it rather than getting a life

------
thrownaway2424
Alternate title: some guy says he was doing the limit in a Porsche.

~~~
timdierks
My clickbait technique is impeccable.

