# Radar **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [October 21, 2005, 12:24pm UTC](https://boards.straightdope.com/t/radar/327243 "2005-10-21T12:24:16Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [October 27, 2005, 1:11am UTC](https://boards.straightdope.com/t/radar/327243/21 "2005-10-27T01:11:08Z")
> [@diogenes009](#): > > You left out one very important aspect of radar and speeding tickets: Radar is a corroborative tool, like pacing and timing between fixed points. At least in NY, a speeding charge requires that a real live human being observe the speeding vehicle, visually estimate the speed of said vehicle and then the officer may or may not corroborate the visual estimate of speed with a corroborative device. The corroborative measure is not necessary to uphold the charge. I’ll have to take your word for it. Not surprisingly, I wrote the article mainly from a technical standpoint rather than a legal one–I am neither qualified for nor interested in such discussion in my Staff Reports. Since the law varies considerably from one location to another, I’d rather not include legal aspects in my articles except in all but the most general of terms. The article was not meant to be a How-To on beating speeding tickets as I’m sure you understand, and as such, the point you raise was unimportant to the general scope of the article. > [@cluth](#): > > A friend told me a while back that some of our newer police cruisers are equipped with laser speed detection systems, which supposedly work much more accurately than the radar-based ones. (He also said that the radar-based ones take a couple of seconds to get a reading, whereas the laser ones are instantaneous). True or not? Laser speed units (called LIDARs) work slightly differently than radars. LIDARs measure speed by directly measuring the return times of pairs of laser pulses and calculating the difference. As such, they can take a reading much more quickly than radars. They are not necessarily more accurate than radars, but they are more targeted–that is, they can pick out a specific vehicle and take a reading on it and it alone, as pointed out earlier. They are not frequently used as yet, however, because they are very expensive and most smaller police departments cannot afford them. That will probably change as the technology becomes cheaper to produce. They have several advantages over radar, such as a greater immunity to spurious return signals and a lower visibility to radar/laser detectors particularly in instant-on applications. However, they are more problematic to operate remotely or in an unattended operation, so radar will always have a niche in traffic enforcement. ---
**Author:** ![danceswithcats](https://avatars.discourse-cdn.com/v4/letter/d/9e8a1a/32.png) [@danceswithcats](https://boards.straightdope.com/u/danceswithcats)\ **Post date:** [October 27, 2005, 2:23am UTC](https://boards.straightdope.com/t/radar/327243/22 "2005-10-27T02:23:17Z")
It’s also worthy of mention that in some states, a margin of error is calculated into the means by which a driver’s speed is sampled before a citation can be issued. For example, in PA a speeding ticket must be for not less than 10 MPH over the posted limit if the means of determination was radar. If VASCAR or ESP strips were used, or plain old time and distance following, a citation may be issued for speeds in excess of 5 MPH over the posted limit. ---
**Author:** ![Flash\_Gordon](https://avatars.discourse-cdn.com/v4/letter/f/df705f/32.png) [@Flash\_Gordon](https://boards.straightdope.com/u/Flash_Gordon)\ **Post date:** [October 27, 2005, 3:26am UTC](https://boards.straightdope.com/t/radar/327243/23 "2005-10-27T03:26:35Z")
> [@DSYoungEsq](#): > > I knew someone would finally post this sort of argument. > ( With all due respect, I just don’t buy your argument. Politics just inevitably gets into this stuff. I definitely do not believe that people look at the posted speed limit and decide they should go faster. I don’t think the posted speed limit means diddly squat to most drivers. When they can, they go the speed that feels safe and comfortable to them. That’s why the 85th percentile rule was used by traffic engineers before speed enforcement became so highly politicized. I live in Denver. It’s one gigantic speed trap. And it is for revenue, only a truly naive person could believe otherwise. All one need do is get a copy of the city budget and see that “fines and penalties” is a huge source of revenue. If a large part of your paycheck depends upon one thing, you will fight to protect that thing, whatever it is. They are used to that money, they are convinced they need it, and they will fight to keep it like a drug addict protecting his stash. Just let the cops stage a slowdown in traffic tickets and see how they scream like stuck pigs. And when the cops do slow down on ticket writing, guess what happens to traffic accidents. Nothing. The number of accidents stays about the same. We have divided parkways with no cross streets and long visibility where the speed limit is 30mph. Just try to go 30 when there are no radar cars and see how everyone behind you will go berserk. But go with the flow, which is about 38-42mph, and all is smooth with no problems. The only problem with traffic on some of these streets is when the cops come and create chaos. The only way to keep a good driving record in this town is to live here long enough that you get to know where the “cherry patches” are, and have a Valentine detector stuck to your windshield. Raza is right, the cops aren’t motivated by the revenue because they don’t get it and it doesn’t affect them. But this this system nevertheless helps them because they don’t have to do traffic enforcement all the time, which would be the pitts I would think. But with this system they can go to a cherry patch and in a few minutes write enough tickets to make their boss think they have been hustling. ---
**Author:** ![DSYoungEsq](https://avatars.discourse-cdn.com/v4/letter/d/c6cbf5/32.png) [@DSYoungEsq](https://boards.straightdope.com/u/DSYoungEsq)\ **Post date:** [October 27, 2005, 3:38am UTC](https://boards.straightdope.com/t/radar/327243/24 "2005-10-27T03:38:38Z")
> [@Flash Gordon](#): > > With all due respect, I just don’t buy your argument. Politics just inevitably gets into this stuff. I definitely do not believe that people look at the posted speed limit and decide they should go faster. I don’t think the posted speed limit means diddly squat to most drivers. When they can, they go the speed that feels safe and comfortable to them. That’s why the 85th percentile rule was used by traffic engineers before speed enforcement became so highly politicized. > > I live in Denver. It’s one gigantic speed trap. And it is for revenue, only a truly naive person could believe otherwise. All one need do is get a copy of the city budget and see that “fines and penalties” is a huge source of revenue. If a large part of your paycheck depends upon one thing, you will fight to protect that thing, whatever it is. They are used to that money, they are convinced they need it, and they will fight to keep it like a drug addict protecting his stash. Just let the cops stage a slowdown in traffic tickets and see how they scream like stuck pigs. And when the cops do slow down on ticket writing, guess what happens to traffic accidents. Nothing. The number of accidents stays about the same. > > We have divided parkways with no cross streets and long visibility where the speed limit is 30mph. Just try to go 30 when there are no radar cars and see how everyone behind you will go berserk. But go with the flow, which is about 38-42mph, and all is smooth with no problems. The only problem with traffic on some of these streets is when the cops come and create chaos. > > The only way to keep a good driving record in this town is to live here long enough that you get to know where the “cherry patches” are, and have a Valentine detector stuck to your windshield. > > Raza is right, the cops aren’t motivated by the revenue because they don’t get it and it doesn’t affect them. But this this system nevertheless helps them because they don’t have to do traffic enforcement all the time, which would be the pitts I would think. But with this system they can go to a cherry patch and in a few minutes write enough tickets to make their boss think they have been hustling. Now, wait a minute. If you agree that individual police officers feel as Raza said, then you cannot believe that cities create speed limits solely for the purpose of mining money from “speeders.” His whole point contradicts the concept. Now think logically. EVERY street has a speed limit. Not every street ends up being monitored for speed purposes. Therefore, on at least some streets, speed limits serve some other purpose. Before you make assertions such as you raise, you might spend some time in a city engineer’s office, or talking to someone from the state’s department of highways. You’ll find out that they take the issue of speed limits quite seriously, for safety reasons, and for reasons of avoiding tort liability. Yeah, there are occasional small towns that still revel in being speed traps, but, in the absence of some better evidence than “look, it just makes sense, the speed limit is so slow and the city makes money off of fines,” you aren’t doing more than whistling Dixie. 😉 ---
**Author:** ![C\_K\_Dexter\_Haven](https://avatars.discourse-cdn.com/v4/letter/c/b2d939/32.png) [@C\_K\_Dexter\_Haven](https://boards.straightdope.com/u/C_K_Dexter_Haven)\ **Post date:** [October 27, 2005, 11:37am UTC](https://boards.straightdope.com/t/radar/327243/25 "2005-10-27T11:37:20Z")
\> DING! DING! DING! \< ::: Moderator rings bell for attention ::: Look, I’ve let this wander far afield because tangents are often interesting, and certainly “how can you fight a ticket if you were caught by radar” is an interesting and relevant tangent. But we’re now strolling along into a debate about the politics behind setting or enforcing speed limits, and that’s too disconnected. If you want to discuss that, start a thread in Great Debates forum. So, please, let’s get back to topic on radar for speeders, and how it works. ---
**Author:** ![Raza](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/raza/32/213_2.png) [@Raza](https://boards.straightdope.com/u/Raza)\ **Post date:** [October 27, 2005, 6:00pm UTC](https://boards.straightdope.com/t/radar/327243/26 "2005-10-27T18:00:57Z")
Sorry…some of the fault for getting off-track is certainly mine. I’ll just comment that, regarding the officer certification and unit testing topics mentioned in the original article, _State of Florida v. Aquilera_ (1979) was absolutely the best thing to happen to speed enforcement since…well, the invention of RADAR. While legally it only affected Dade County, it was watched nationally, and helped usher in a set of standard procedures and laws in many states designed to protect the motoring public from haphazard speed RADAR use. ---
**Author:** ![picunurse](https://avatars.discourse-cdn.com/v4/letter/p/7bcc69/32.png) [@picunurse](https://boards.straightdope.com/u/picunurse)\ **Post date:** [October 29, 2005, 4:15am UTC](https://boards.straightdope.com/t/radar/327243/27 "2005-10-29T04:15:24Z")
> [@cluth](#): > > A friend told me a while back that some of our newer police cruisers are equipped with laser speed detection systems, which supposedly work much more accurately than the radar-based ones. (He also said that the radar-based ones take a couple of seconds to get a reading, whereas the laser ones are instantaneous). True or not? > > Another friend told me recently that some cops are “traffic cops” and others aren’t. He said that the non-traffic cops won’t pull you over even for blatant traffic violations (such as running a red light). I’d never heard of such a thing, but this friend is an, er, professional speeder. Can anyone corroborate this? > > depireux–wouldn’t the continuum of speeds be from 0 to double the car’s speed? (The bottom-most spoke has a relative velocity of 0 for an instantaneous moment, while the top-most spoke is moving at double the car’s velocity.) I know that if there’s a person in the back seat of a patrol car, they won’t stop even if your hair were on fire. ---
**Author:** ![coop](https://avatars.discourse-cdn.com/v4/letter/c/87869e/32.png) [@coop](https://boards.straightdope.com/u/coop)\ **Post date:** [October 30, 2005, 3:33am UTC](https://boards.straightdope.com/t/radar/327243/28 "2005-10-30T03:33:47Z")
Does a hand-held laser read accurately when it is raining? Would the beam be dispersed by the drops of water, hence, the harder it is raining, the less accurate the reading would be? ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [October 30, 2005, 4:23am UTC](https://boards.straightdope.com/t/radar/327243/29 "2005-10-30T04:23:38Z")
Rain doesn’t affect the accuracy of either radar or LIDAR to any significant degree, but it can impact the _range_ quite a bit. ---
**Author:** ![coop](https://avatars.discourse-cdn.com/v4/letter/c/87869e/32.png) [@coop](https://boards.straightdope.com/u/coop)\ **Post date:** [October 30, 2005, 5:46am UTC](https://boards.straightdope.com/t/radar/327243/30 "2005-10-30T05:46:32Z")
I understand that it would impact the further the beam had to travel, but if the rain was heavy, would not each drop tend to act like a prism and disperse the beam??? Much the same principle as a rainbow… ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [October 30, 2005, 4:29pm UTC](https://boards.straightdope.com/t/radar/327243/31 "2005-10-30T16:29:04Z")
More or less, but that will only serve to reduce the effective range, since some of the signal will be scattered away from either the target or the receiver. Accuracy depends on the signal being returned to the receiver, and in the case of radar, this return signal is very small compared to the return signal of even a small vehicle. As indicated in the article, the receivers are usually capable of taking readings on either fastest-moving return or the strongest return, neither of which will be that returned by intervening raindrops. Laser is a bit different, since the drops _will_ individually return a very small portion of the incoming laser light straight back to the receiver by total internal reflection, as I stated earlier. However, these returns will show up as a series of very tiny spikes in the return data, followed by a much larger spike from the target vehicle, so again, it’s very easy for the equipment to ignore. In both cases, range is the only factor which is greatly affected, even in very heavy rains. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 31, 2005, 12:31am UTC](https://boards.straightdope.com/t/radar/327243/32 "2005-10-31T00:31:39Z")
You could probably approximate the effective range of laser in a storm by how far you can see clearly. Water will have much the same effect on the laser as on the light you’re seeing. ---
**Author:** ![eviladam\_part\_2](https://avatars.discourse-cdn.com/v4/letter/e/da6949/32.png) [@eviladam\_part\_2](https://boards.straightdope.com/u/eviladam_part_2)\ **Post date:** [November 3, 2005, 4:04pm UTC](https://boards.straightdope.com/t/radar/327243/33 "2005-11-03T16:04:30Z")
> [@Chronos](#): > > It’s technically possible to jam radar, but it’s very easy to determine the source of the jamming. And I believe that in most jurisdictions, the jamming is itself illegal, and probably carries a stiffer penalty than the speeding would have in the first place. Add to that the fact that the cop has other methods of determining your speed, not all of which can be jammed (it would be exceedingly difficult to jam a laser, and impossible to jam pacing, reference marks, or visual estimation), and a radar jammer turns out to be a phenomenally bad idea. to elaborate on this a little more, radar jamming worked in the olden days, when radar guns were slower and used a select few frequencies. modern radar guns use a wide variety of frequencies, and are now fast enough that by the time your jammer detects the incoming signal and figures out what frequency to jam, it’s too late. all in all it’s highly illegal, easily detectable, and ineffective. as an interesting sidenote, laser jamming is actually possible and generally legal because the frequency is not regulated by the FCC. ---
**Author:** ![slondeau](https://avatars.discourse-cdn.com/v4/letter/s/e68b1a/32.png) [@slondeau](https://boards.straightdope.com/u/slondeau)\ **Post date:** [November 3, 2005, 4:15pm UTC](https://boards.straightdope.com/t/radar/327243/34 "2005-11-03T16:15:08Z")
> [@eviladam](#): > > to elaborate on this a little more, radar jamming worked in the olden days, when radar guns were slower and used a select few frequencies. modern radar guns use a wide variety of frequencies, and are now fast enough that by the time your jammer detects the incoming signal and figures out what frequency to jam, it’s too late. all in all it’s highly illegal, easily detectable, and ineffective. > > as an interesting sidenote, laser jamming is actually possible and generally legal because the frequency is not regulated by the FCC. EvilAdam is correct, imo. The days of Radar jamming has past I think. Most officers use Laser these days anyway. Buy a laser jammer, and you will be OK. The Phaser is supposed to work pretty good! ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [November 3, 2005, 4:26pm UTC](https://boards.straightdope.com/t/radar/327243/35 "2005-11-03T16:26:13Z")
> [@eviladam](#): > > [A]s an interesting sidenote, laser jamming is actually possible and generally legal because the frequency is not regulated by the FCC. The FCC is not the only governing body in the US, you know. The State of California disagrees with you. See Section 28150 of the CVC: > [@](#): > > **Jamming: Electronic Speed-Measuring Devices** > > 1. (a) No vehicle shall be equipped with any device that is designed for, or is capable of, jamming, scrambling, neutralizing, disabling, or otherwise interfering with radar, **laser** , or any other electronic device used by a law enforcement agency to measure the speed of moving objects. > > (b) No person shall use, buy, possess, manufacture, sell, or otherwise distribute any device that is designed for jamming, scrambling, neutralizing, disabling, or otherwise interfering with radar, **laser** , or any other electronic device used by a law enforcement agency to measure the speed of moving objects. > > (c) Except as provided in subdivision (d), a violation of subdivision (a) or (b) is an infraction. > > (d) When a person possesses four or more devices in violation of subdivision (b), the person is guilty of a misdemeanor. > > (e) Notwithstanding any other provision of law, a person who has a valid federal license for operating the devices described in this section may transport one or more of those devices if the license is carried in the vehicle transporting the device at all times when the device is being transported. Other states may have similar laws, so be warned. Emphasis added. ---
**Author:** ![slondeau](https://avatars.discourse-cdn.com/v4/letter/s/e68b1a/32.png) [@slondeau](https://boards.straightdope.com/u/slondeau)\ **Post date:** [November 3, 2005, 4:34pm UTC](https://boards.straightdope.com/t/radar/327243/36 "2005-11-03T16:34:00Z")
> [@Q.E.D.](#): > > The FCC is not the only governing body in the US, you know. The State of California disagrees with you. See Section 28150 of the CVC: > > Other states may have similar laws, so be warned. > > Emphasis added. that is true. Michigan, does not have those laws that I am aware of. 😃 Yay for Michigan. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 3, 2005, 6:37pm UTC](https://boards.straightdope.com/t/radar/327243/37 "2005-11-03T18:37:41Z")
IANAL. But IAAPhysicist. So I won’t get into the legalities of a laser jammer. However, I fail to see how a laser jammer could be feasibly implemented. You’d need for your entire car to be glowing at a range of frequencies around the laser frequency (or rather, all of the laser frequencies used by any model of lidar gun), significantly brighter than the brightness of the laser illumination, and it’d need to either be always on, or turn on quicker than the lidar can get a fix. In principle, all of this is possible, but the power requirements would be immense, and I’d be worried about cooking the passengers. ---
**Author:** ![eviladam\_part\_2](https://avatars.discourse-cdn.com/v4/letter/e/da6949/32.png) [@eviladam\_part\_2](https://boards.straightdope.com/u/eviladam_part_2)\ **Post date:** [November 3, 2005, 9:59pm UTC](https://boards.straightdope.com/t/radar/327243/38 "2005-11-03T21:59:31Z")
i should have emphasized the “generally” in my “generally legal” statement. check all state and local laws, blah blah. > [@Chronos](#): > > IANAL. But IAAPhysicist. So I won’t get into the legalities of a laser jammer. However, I fail to see how a laser jammer could be feasibly implemented. You’d need for your entire car to be glowing at a range of frequencies around the laser frequency (or rather, all of the laser frequencies used by any model of lidar gun), significantly brighter than the brightness of the laser illumination, and it’d need to either be always on, or turn on quicker than the lidar can get a fix. In principle, all of this is possible, but the power requirements would be immense, and I’d be worried about cooking the passengers. laser jammers typically have a detector that is mounted near the primary reflective target areas (front license plate, headlights). as soon as the unit detects laser, it fires a several second burst of LEDs (mounted on the license plate frame or grill) to confuse the reading, and sound a warning alert to the driver. the driver slows down, and seconds later the laser gun is able to get a reading of the now-legal speed. i think laser is slower at locking in a reading because it doesn’t measure the doppler-shifted radar frequency, it uses a series of distance measurements averaged over a certain time to compute speed. ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [November 3, 2005, 11:19pm UTC](https://boards.straightdope.com/t/radar/327243/39 "2005-11-03T23:19:21Z")
The Radar unit has to measure the beat frequency between the outgoing and return signal…this is a fairlly low frequency signal. Accuracy of measuring a frequency depends on the time taken to measure it. The time can be reduced by measuring the period, however, due to noise, several averaged measurments are still required. LIDAR pulses are pretty easy to pick out of the noise. If the two intervals between three pulses agree on the infered speed, you probably have a good reading, a fourth pulse will confirm it. In human reaction terms, this requires less than an eyeblink. The net result is that LIDAR can give an accurate speed reading slightly faster than radar. And yes, infrared LEDs and even diode lasers are cheap and compact compared to gunn diodes (or dog forbid magnatrons) needed to generate microwave jamming. Also, the LIDAR units are not too picky about the exact wavelength as long as it is close, whereas jamming radar requires pretty good frequency accuracy and stability (or lots of power). ---
**Author:** ![ReuvenB](https://avatars.discourse-cdn.com/v4/letter/r/9fc348/32.png) [@ReuvenB](https://boards.straightdope.com/u/ReuvenB)\ **Post date:** [November 4, 2005, 12:42am UTC](https://boards.straightdope.com/t/radar/327243/40 "2005-11-04T00:42:45Z")
> [@Chronos](#): > > IANAL. But IAAPhysicist. So I won’t get into the legalities of a laser jammer. However, I fail to see how a laser jammer could be feasibly implemented. You’d need for your entire car to be glowing at a range of frequencies around the laser frequency (or rather, all of the laser frequencies used by any model of lidar gun), significantly brighter than the brightness of the laser illumination, and it’d need to either be always on, or turn on quicker than the lidar can get a fix. In principle, all of this is possible, but the power requirements would be immense, and I’d be worried about cooking the passengers. Or you could paint the car matte black. [Previous page](https://boards.straightdope.com/t/radar/327243.md?page=1) [Next page](https://boards.straightdope.com/t/radar/327243.md?page=3)