Radar

[diogenes009]

In reference to the following article:

Radar (http://www.straightdope.com/mailbag/mradar.html) Radar (http://www.straightdope/mailbag/mradar.html)

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.

Wait a minute, I hear you say. Do I mean to say that some Barney Fife’s WAG of my speed is enough to get a ticket? Well, yes. But it isn’t as preposterous as it sounds.

The standard set by the courts for establishing that a person is an expert in speed visual estimation is that they can visually estimate the speed of twenty vehicles within +/- 5 mph. This superhuman feat is part of the radar certification process in NY and probably many other jurisdictions. Truthfully though, give me fifteen minutes of your time, and a working radar unit, and I’ll have you doing it like a pro.

The reason for this convention is simple. The courts here have determined that it is impossible to cross examine a radar device. An inanimate object tends not to answer questions very well.

So a live human being, making a reasonable judgment using a simple and readily acquired skill is necessary to bring a charge. Further, a real live operator, aware of and correcting for the limitations of the radar device is able to ensure that erroneous readings do not become the basis for a ticket.

I, for one, am more comfortable with that than the idea that some piece of hardware, unattended and unsupervised is lifting cash from my wallet and putting points on my license.

[Nature's Call]

I, for one, am more comfortable with that than the idea that some piece of hardware, unattended and unsupervised is lifting cash from my wallet and putting points on my license.
Thanks you two (diogenes009 and QED) While I've never had the balls to fight a ticket, I've fantasized cross-examining the cop, asking at what angle did the reading take place hoping he'd stumble and I'd get off. Now I see that the only effect of the angle is to lower the reading, and the cop's gonna be believed without the device anyways...

Now I have to find something else to fantasize about where I get off.

[depireux]

One thing I wonder about radar detectors is the fact that the top of the wheels of a moving car goes at twice the speed of the car. No big deal, except that some wheels have triangularly shaped spokes that are apt to reflect microwaves quite well, possibly better than the rest of the car which might be mostly non-metallic (like an older corvette) and have strongly reflecting surfaces (like the radiator) angled in such a way it does not reflect the radar beam well. So depending on the part of the wheel the radar beam reflects off of, you might get a continuum of speed readings between the actual speed and twice that speed.

[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.)

[depireux]

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.)

Ah, good point. The wheel spokes would provide a continuous background, which hopefully would not be detected as a discrete peak in the spectrum of possible doppler shifters.

[cluth]

Ah, good point. The wheel spokes would provide a continuous background, which hopefully would not be detected as a discrete peak in the spectrum of possible doppler shifters.

I suppose it would depend on how narrow the Ka-band radar beam is at the car's distance. 1 or 2cm might be narrow enough to focus on just one or two of the spokes of your previously-mentioned car. If it's more like a couple of feet wide, then the reflection from the car itself would probably overwhelm the small peak from the spokes, and you'd be correct.

[diogenes009]

First of all, radar reads the greatest signal return or the fastest speed from the larger targets in some models. Individual components of the vehicle that may be creating a higher return will not be sufficiently reflective or exposed to be of any moment. One of the reasons an operator should be part of the speed enforcement equation is that an operator can identify and disregard spurious readings. Left to its’ own devices, a radar unit can give some quite disconcerting results.

As to Ladar, or laser speed measurement tools, they are becoming more common. Laser has the advantage of a smaller target area, resulting in the ability to say “That blue car is going x mph.” as opposed to “A car in that cluster is doing x mph, and based upon my training and expertise, it is the blue one.”

Radar comes in two primary packages. First there is a dash mounted model which you stick there with brackets of Velcro. The antenna is mounted on the dash, the visor or on the outside of the vehicle. The second common type is the funny looking gun type that we are familiar with. This type of package is the only type laser comes in, because you have to actually aim laser at a car as opposed to pointing it in a direction as in radar.

Also, a police car is a commercial automobile with or without added frame, suspension and powertrain hardening for heavy duty use. They also have fewer luxury features, no carpet, AM/FM only radios, etc. The agency buying the vehicle gets a fleet discount on these models and then either has the vender add radios, lights, cages, etc. or does it themselves. Many jurisdictions simply use regular stripper versions of cars right off the same lot you bought yours at.

As to traffic cops and regular cops, many cities have special traffic units that do traffic enforcement as their primary or only task. Unlike most parking bureaus, though, traffic squads are manned with real fully empowered cops. They can pinch you for any offense observed should they care to.

The reason for traffic squads is that in a busy city the patrol officers are running from complaint to complaint and do not have the time to stop you for petty traffic offenses, though they do have the authority.

Finally, to the idea of fighting a speeding ticket. When you are charged with any offense, the burden is on the prosecution to establish beyond reasonable doubt that the defendant committed the elements of the offense. In a speeding trial the prosecutor has to establish that: 1) The defendant was operating the motor vehicle, 2) That the vehicle was operated in excess of a properly posted speed limit. (Signage or other legally required notice properly in place. The limit properly filed as law by the jurisdiction.) 3) That the method of speed detection was proper. (Visual estimation, certification of corroborative devices, etc.)

As the defendant you need to challenge the elements of the offense. You will get nowhere arguing about fairness, what a great guy you are, how busy you are, what a jerk the officer was or that you were rushing to get home to use the bathroom. You have to focus on the elements of the offense. A common mistake is that people get upset and stop thinking. Remember, the officer existed before you first saw him. He may have had to go some distance to catch up with you and find a safe place to pull you over. Just because you were speeding on one street doesn’t mean you have to be stopped on that street, let alone in that jurisdiction.

I respect people who have the courage to fight a ticket. I just wish they had a better idea of what they were doing.

[cluth]

diogenes009--Many thanks for sharing your knowledge and answering my questions.

Although you said that the smaller parts which could give false readings aren't really of any worry, do you know what the angular width of the Ka-band radars used is? On average, how far away do traffic radars usually measure speed? (And consequently, how wide is the average beam when it hits my car?)

[diogenes009]

diogenes009--Many thanks for sharing your knowledge and answering my questions.

Although you said that the smaller parts which could give false readings aren't really of any worry, do you know what the angular width of the Ka-band radars used is? On average, how far away do traffic radars usually measure speed? (And consequently, how wide is the average beam when it hits my car?)

Radar generally is the same regardless of band. The antenna emits a cigar shaped signal path that goes quite far. I’m a bit rusty on specs, but I believe it can give effective readings out to a mile or more in open terrain. At that distance the antennas aim would need to be pretty good. You’d be getting to the thin end of the cigar. Where most radar is done is in the quarter mile range or less, where the beam would easily splash over your vehicle and the ones around it. That is one of the lasers advantages, the beam width is a couple feet, point it at the car you want a speed for.

[Master_Blaster]

What about these so-called RADAR jammers that I have heard and read about lately? These things supposedly jam the radar cop's signal so that he either gets no reading or a false reading; thus allowing the speeding to go free.

Do these things really work? And if so, how?

[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.

[DSYoungEsq]

And that's without getting into the question of whether or not the Board will allow you to talk about how to break the law. ;)

I liked the article by Q.E.D. It was informative. It wasn't preachy, or filled with assumptions. I suppose it could have focussed more upon the practical aspects of radar units, rather than a lengthy explanation of doppler effect speed measurement, but still, a good article.

I've never understood those who think it is somehow "unfair" that the police resort to radar to establish speed limit violations... :dubious:

[Nature's Call]

I've never understood those who think it is somehow "unfair" that the police resort to radar to establish speed limit violations... :dubious:
Radar detectors are legal in Alberta (or at least they were 10 years ago when I lived there). Instead of radar, there are lines painted on the highway perpendicualr to the direction of travel. These lines are such-and-so distance apart. Cops in airplanes time how long it takes you to reach the next line, then radio down to the waiting cop with your vehicle's description (probably licence plate too).

[ftg]

I've always heard that the other actors were really happy when he left the show.

What?

Oh, nevermind.

[Mathochist]

I, for one, am more comfortable with that than the idea that some piece of hardware, unattended and unsupervised is lifting cash from my wallet and putting points on my license.

What about calculus? Any competent calc 1 student can tell you about the differential mean value theorem. If a toll road (like, say, the New Jersey Turnpike) were to synchronize its clocks and mark the time in and time out of any vehicle, measure that against the distance between entry and exit points, and determine an average speed of greater than the maximum speed limit on the thoroughfare (hey, give it +10mph margin of error if you want), then we know that at some point the vehicle was exceeding that speed. Would you have the same complaint with that automated method?

[diogenes009]

What about calculus? Any competent calc 1 student can tell you about the differential mean value theorem. If a toll road (like, say, the New Jersey Turnpike) were to synchronize its clocks and mark the time in and time out of any vehicle, measure that against the distance between entry and exit points, and determine an average speed of greater than the maximum speed limit on the thoroughfare (hey, give it +10mph margin of error if you want), then we know that at some point the vehicle was exceeding that speed. Would you have the same complaint with that automated method?

Isn't the real question: Why haven't they done so already? I suspect they fear a significant loss of toll revenue if they slow the traffic speed to the point where other routes are more attractive. Besides, with the small amount of time saved per trip by speeding to the extent most people do so, a pit stop for fluid exchange would move most motorists back into the legal time frame.

And no, since the time/distance scenario is less fraught with error than speed measurements using radar, it wouldn't bother me as much. However, I would like to point out that each time I get my Easypass statement I have to call and dispute at least one toll charge. There is a difference between theory and successful execution.

[Mathochist]

Isn't the real question: Why haven't they done so already? I suspect they fear a significant loss of toll revenue if they slow the traffic speed to the point where other routes are more attractive.

Largely it's because of a tremendous public antipathy to the encroachment of their "right to speed". The voters in NJ wouldn't stand for it.

[Flash Gordon]

Finally, to the idea of fighting a speeding ticket. When you are charged with any offense, the burden is on the prosecution to establish beyond reasonable doubt that the defendant committed the elements of the offense. In a speeding trial the prosecutor has to establish that: 1) The defendant was operating the motor vehicle, 2) That the vehicle was operated in excess of a properly posted speed limit. (Signage or other legally required notice properly in place. The limit properly filed as law by the jurisdiction.) 3) That the method of speed detection was proper. (Visual estimation, certification of corroborative devices, etc.)

As the defendant you need to challenge the elements of the offense. You will get nowhere arguing about fairness, what a great guy you are, how busy you are, what a jerk the officer was or that you were rushing to get home to use the bathroom. You have to focus on the elements of the offense. A common mistake is that people get upset and stop thinking. Remember, the officer existed before you first saw him. He may have had to go some distance to catch up with you and find a safe place to pull you over. Just because you were speeding on one street doesn’t mean you have to be stopped on that street, let alone in that jurisdiction.

I respect people who have the courage to fight a ticket. I just wish they had a better idea of what they were doing.

The problem with fighting speeding tickets is that in most cases you're guilty and fighting it just prolongs the agony. The real problem, it seems to me, is that bureaucrats and politicians use traffic enforcement for revenue. In order to maximize revenue posted speed limits are often set about 10 miles below what a reasonable speed for that road would be, assuming a reasonable speed is that which about 85% of cars on that roadway are traveling at any given time.

I think instead of fighting the radar gods a better way for citizens to spend their time is to let their elected representatives know they don't approve of using traffic enforcement as a revenue measure and want it changed.

[DSYoungEsq]

The problem with fighting speeding tickets is that in most cases you're guilty and fighting it just prolongs the agony. The real problem, it seems to me, is that bureaucrats and politicians use traffic enforcement for revenue. In order to maximize revenue posted speed limits are often set about 10 miles below what a reasonable speed for that road would be, assuming a reasonable speed is that which about 85% of cars on that roadway are traveling at any given time.

I think instead of fighting the radar gods a better way for citizens to spend their time is to let their elected representatives know they don't approve of using traffic enforcement as a revenue measure and want it changed.
I knew someone would finally post this sort of argument.

Despite what many would like to believe, with the exception of a few usually well-known locations, city and county (not to mention state) jurisdictions post speed limits on the basis of safety issues, not revenue desires. They have a whole kit and kaboodle of criteria for determining such things, just like they do for deciding whether or not to put a traffic control device in, etc. They have developed these criteria as a response to tort lawsuits which find liability on the part of the governmental entity for failing to approach safety issues in an appropriate way.

The reason that most people go faster than the speed limit isn't that the limit is usually inappropriately slow. Rather, it is because we are conditioned to think that speed limits should be exceeded, without any sense of guilt in the behaviour. When we see "45" we think "55," safe in the "knowledge" that there must first of all be some sort of "fudge safety factor" built into the speed limit (that is, it can't be set at the highest safe speed because that would open the government to potential liability), and second of all that traffic control units allow a certain leeway (usually 5-10 mph) before they get upset at you and ticket you. In our fast-paced society, the concept of actually limiting ourselves on the basis of what someone else, especially the government, wants us to do is most repugnant.

You have the issue of speed limits quite backwards, I am afraid. Indeed, in California, you are specifically backward: one of the criteria for setting a limit higher than it is already set is a traffic survey showing that a significant percentage (I forget the exact amount) travels at a rate of speed 10 MPH or more than the posted limit. Traffic along California Hwy. 1 near Santa Cruz was surveyed for a potential limit increase in the 80's or 90's while I lived near there. Why is this a criterion? Because it is seen as a potential safety issue if you post a limit that slows the few law-abiding citizens down significantly compared to traffic flow.

Near where I live, there is a road that connects two main E-W roads. It is mildly curvy, deceptively so, actually. It is posted 35; almost no one does that speed. People complained about it all the time to the township and county; accusations of revenue enhancement were common. Well, they were, that is, until the teenager crashed and killed himself by driving a mere 10 mph over the posted limit, in good weather, during the day. :(

[Raza]

Several have referred to the revenue-generation aspects of speeding tickets; I don't refute that cities enjoy that extra revenue. However, except perhaps in the smallest of towns, individual officers do not factor that in to their activities at all - the revenue realized from a ticket is so far removed from the officer that he perceives no direct benefit.

I believe the bigger problem with speed enforcement in general lies with the "quota" factor. Let me be clear: never ever have I been told that I must or should write X number of tickets per shift or per month (on the other hand, I've never worked more than a few days on a traffic squad). Regardless, though, there is a belief that writing some citations shows some initiative and at least a base-level of proactive enforcement. It just so happens that writing speeding and expired-tag citations are the quickest, simplest, and most straightforward. Thus, an officer can take out a RADAR gun for a day and get half-a-month's expected citation output.

Where does that officer go to run RADAR? Why, the streets that have the highest number of speeders per minute. It seldom (I'm generalizing a lot here) has to do with speed-related danger - it is about easy enforcement. Unless acting on citizen complaints, most officers will go to where to pickin's are easiest. No officer wants to sit idle for an hour waiting for that one hotshot.

Without question speed increases the severity of an accident, but it is seldom the cause of an accident. I truly believe we need to put away our RADAR guns and start cracking down on aggressive driving. In my area, it is common for cars to follow one-car-length behind at 75mph, and for cars to cross four lanes of traffic in 1/10 of mile (without signalling, of course). I believe these are much greater causes of serious accidents than simply speeding.

The author mentioned that both officers and devices are certified and that devices are regularly retested. That is certainly true in the state where I worked, and I hope it is now nationwide, but I am not certain. Certainly officers can make mistakes (I always erred on the side of non-action if in doubt), and in heavy traffic situations it is frighteningly easy. Until vehicle "black boxes" become common, I don't know of any reliable way to prevail in court if you are the victim of such a mistake. The advisability and practicality of fighting a ticket varies widely based on local practices and judges. For us, sometimes the officer can't make it to court, allowing an easy defendant victory. For some judges, it isn't difficult to make a reasoned argument that casts just enough doubt in the judge's mind to win. Other judges, though, won't hear of it.

One thing I have found, though, right or wrong: having an attorney with you strongly tilts things in your favor, even if the attorney doesn't have much to offer in the way of defense.

[Q.E.D.]

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.
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.

[danceswithcats]

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.

[Flash Gordon]

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.

[DSYoungEsq]

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. ;)

[C K Dexter Haven]

> 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.

[Raza]

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.

[picunurse]

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.

[coop]

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?

[Q.E.D.]

Rain doesn't affect the accuracy of either radar or LIDAR to any significant degree, but it can impact the range quite a bit.

[coop]

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...

[Q.E.D.]

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.

[Chronos]

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.

[eviladam, part 2]

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.

[slondeau]

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!

[Q.E.D.]

[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

28150. (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.

[slondeau]

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. :D Yay for Michigan.

[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.

[eviladam, part 2]

i should have emphasized the "generally" in my "generally legal" statement. check all state and local laws, blah blah.

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.

[Kevbo]

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).

[ReuvenB]

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.

[Q.E.D.]

Or you could paint the car matte black.
Won't help much. The important reflective parts are the retroflective license plates, the corner cube arrays in the taillights, turn signals, other marker lamps/reflectors and the headlights. Reflections from the body are going to be small in relation to those from the above mentioned sources.

[eviladam, part 2]

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).

this link (http://www.pbelectronics.com/police_lidar_laser.htm) says laser requires ~0.3 seconds to take a speed measurement. the extremely narrow beam of the radar requires more averaging, to eliminate errors from possibility of one pulse hitting the front of the car, and the next pulse reflecting off the windshield. since it's a calculation based on distance measurements, if the measured signal is coming from a different point it will cause huge errors.


this link (http://www.stalkerradar.com/sports_sport.shtml) has a radar gun that requires 0.046 seconds to measure speed.

traffic radar is not a "fairly low frequency". X band (mostly obsolete) is ~10GHz, and the more popular K and Ka are ~24GHz and ~33GHz.

[Q.E.D.]

traffic radar is not a "fairly low frequency". X band (mostly obsolete) is ~10GHz, and the more popular K and Ka are ~24GHz and ~33GHz.
He doesn't mean the carrier signal frequency, he means the beat frequency--the differential between the outgoing carrier and the Doppler-shifted return signal. This can be only a few hertz. That's what he meant by "fairly low frequency".