question: How do cops get your speed while sitting in their vehicles and not having to use a radar gun to point and shoot?
comment: Many have mounted units the car. They just adjust it to point in the direction of traffic they are monitoring. Many can register in both directions.
passages: BULLET::::- Vehicle tracking system: Some police vehicles, especially traffic units, may be fitted with equipment which will alert the officers to the presence nearby of a stolen vehicle fitted with a special transponder, and guide them towards it, using GPS or simpler radio triangulation.
   Since this type of speed gun measures the difference in speed between a target and the gun itself, the gun must be stationary in order to give a correct reading. If a measurement is made from a moving car, it will give the difference in speed between the two vehicles, not the speed of the target relative to the road, so a different system has been designed to work from moving vehicles.
  
  Section::::How it works.:Moving radar.
   Some sophisticated devices may produce different speed measurements from multiple objects within the field of view. This is used to allow the speed-gun to be used from a moving vehicle, where a moving and a stationary object must be targeted simultaneously, and some of the most sophisticated units are capable of displaying up to four separate target speeds while operating in moving mode once again emphasizing the importance of the operators' ability to consistently and accurately visually estimate speed.
  
  Section::::Limitations.:Environment.
   However, there are a number of limitations to the use of radar speed guns. For example, user training and certification are required so that a radar operator can use the equipment effectively, with trainees being required to consistently visually estimate vehicle speed within +/-2 mph of actual target speed, for example if the target's actual speed is 30 mph than the operator must be able to consistently visually estimate the target speed as falling between 28 and 32 mph. Stationary traffic enforcement radar must occupy a location above or to the side of the road, so the user must understand trigonometry to accurately estimate vehicle speed as the direction changes while a single vehicle moves within the field of view. Actual vehicle speed and radar measurement thus are rarely the same, however, for all practical purposes this difference in actual speed and measured speed is inconsequential, generally being less than 1 mph difference, as police are trained to position the radar to minimize this inaccuracy and when present the error is always in the favor of the driver reporting a lower than actual speed. Radar speed guns do not differentiate between targets in traffic, and proper operator training is essential for accurate speed enforcement. This inability to differentiate among targets in the radar's field of view is the primary reason for the operator being required to consistently and accurately visually estimate target speeds to within +/-2 mph, so that, for example if there are seven targets in the radar's field of view and the operator is able to visually estimate the speed of six of those targets as approximately 40 mph and visually estimate the speed of one of those targets as approximately 55 mph and the radar unit is displaying a reading of 56 mph it becomes clear which target's speed the unit is measuring.
   Section::::Newer instruments.
  
  Laser devices, such as a LIDAR speed gun, are capable of producing reliable range and speed measurements in typical urban and suburban traffic environments without the site survey limitation and cameras. This is reliable in city traffic because LIDAR has directionality similar to a typical firearm because the beam is shaped more like a pencil that produces measurement only from the object it has been aimed at.
  
  Section::::In media.
  
  MythBusters did an episode on trying to get the gun to have incorrect readings by changing the surface of the passing object.
  
  Section::::See also.
  
  BULLET::::- LIDAR detector
   BULLET::::- Speed recognition device: Some police cars are fitted with devices to measure the speed of vehicles being followed, such as ProViDa, usually through a system of following the vehicle between two points a set distance apart. This is separate to any radar gun device which is likely to be handheld, and not attached to the vehicle.
  
  BULLET::::- Remote rear door locking: This enables officers in the front to remotely control the rear locks—usually used in conjunction with a transport enclosure.
   1947, the system was tested by the Connecticut State Police in Glastonbury, Connecticut, initially for traffic surveys and issuing warnings to drivers for excessive speed. Starting in February 1949, the state police began to issue speeding tickets based on the speed recorded by the radar device. In 1948, radar was also used in Garden City, New York.
  
  Section::::How it works.
  
  Section::::How it works.:Doppler effect.
  
  Speed guns use Doppler radar to perform speed measurements.
   In so-called "moving radar", the radar antenna receives reflected signals from both the target vehicle and stationary background objects such as the road surface, nearby road signs, guard rails and streetlight poles. Instead of comparing the frequency of the signal reflected from the target with the transmitted signal, it compares the target signal with this background signal. The frequency difference between these two signals gives the true speed of the target vehicle.
  
  Section::::How it works.:Design considerations.
  
  Modern radar speed guns normally operate at X, K, K, and (in Europe) K bands.
   A second limitation for hand-held devices is that they have to use continuous-wave radar to make them light enough to be mobile. Speed measurements are only reliable when the distance at which a specific measurement has been recorded is known. Distance measurements require pulsed operation or cameras when more than one moving object is within the field of view. Continuous-wave radar may be aimed directly at a vehicle 100 yards away but produce a speed measurement from a second vehicle 1 mile away when pointed down a straight roadway. Once again falling back on the training and certification requirement for consistent and accurate visual estimation so that operators can be certain which object's speed the device has measured without distance information, which is unavailable with continuous wave radar.
   This is a typical problem with radar speed guns used by law enforcement officers, NASCAR events, and sports, like baseball, golf, and tennis. Interference from a second radar, automobile ignition, other moving objects, moving fan blades on the intended target, and other radio frequency sources will corrupt measurements. These systems are limited by wavelength, which is 0.02 meter at Ku band, so the beam spread exceeds 45 degrees if the antenna is smaller than 12 inches (0.3 meter). Significant antenna side-lobes extend in all directions unless the antenna is larger than the vehicle on which the radar is mounted.
   where "c" is the speed of light, "f" is the emitted frequency of the radio waves and Δ"f" is the difference in frequency between the radio waves that are emitted and those received back by the gun. This equation holds precisely only when object speeds are low compared to that of light, but in everyday situations, this is the case and the velocity of an object is directly proportional to this difference in frequency.
  
  Section::::How it works.:Stationary radar.
   In recent years, some radar detectors have added GPS technology. This allows users to manually store the locations where police frequently monitor traffic, with the detector sounding an alarm when approaching that location in the future (this is accomplished by pushing a button and doesn't require coordinates to be entered). These detectors also allow users to manually store the coordinates of sites of frequent false alarms, which the GPS enabled detector will then ignore. The detector can also be programmed to mute alerts when traveling below a preset speed, limiting unnecessary alerts. Some GPS enabled detectors can download the GPS coordinates of speed monitoring cameras and red-light cameras from the Internet, alerting the driver that they are approaching the camera.
   Speed is the change in distance to an object with respect to time. Thus the existing system for measuring distance, combined with a memory capacity to see where the target last was, is enough to measure speed. At one time the memory consisted of a user making grease pencil marks on the radar screen and then calculating the speed using a slide rule. Modern radar systems perform the equivalent operation faster and more accurately using computers.
   Virginia is the only state that prohibits the use of radar detectors (the District of Columbia does as well, though it is not a state).
  
  Section::::Wake Island.
  
  The speed limit on Wake Island is 40 mph (64 km/h).
  
  Section::::Washington.
   In terms of judging a person's speed, the best method of timing is through lasers which start and stop the times when passed through. A laser start (from a stationary position) is more accurate for measuring pure speed as it does not register a runner's reaction time, however, this method of timing a 40-yard dash can affect the accuracy by as much as 0.5 seconds with the manual stopwatch method. 
   Cameras are associated with automated ticketing machines (known in the UK as speed cameras) where the radar is used to trigger a camera. The radar speed threshold is set at or above the maximum legal vehicle speed. The radar triggers the camera to take several pictures when a nearby object exceeds this speed. Two pictures are required to determine vehicle speed using roadway survey markings. This can be reliable for traffic in city environments when multiple moving objects are within the field of view. It is the camera, however, and its timing information, in this case, that determines the speed of an individual vehicle, the radar gun simply alerting the camera to start recording.
   BULLET::::- Pursuit; Chase down the suspect. You arrest him by either shooting his car, totaling it, or using the PIT maneuver (hitting the back-end at the side to put the suspect into a spin.) and stopping in front of him. For some specific missions, you can use the drone to destroy the suspect vehicle.
   Traffic radar comes in many models. Hand-held units are mostly battery powered, and for the most part are used as stationary speed enforcement tools. Stationary radar can be mounted in police vehicles and may have one or two antennae. Moving radar is employed, as the name implies, when a police vehicle is in motion and can be very sophisticated, able to track vehicles approaching and receding, both in front of and behind the patrol vehicle and also able to track multiple targets at once. It can also track the fastest vehicle in the selected radar beam, front or rear.
   In the UK, the evidence is only admissible in court if the speed measured by the radar and the speed calculated from the distance travelled between the photographs agrees within 10%. The speed indicated by the radar unit is too unreliable to be used as the sole means of evidence as it is prone to error due to multiple reflections etc. Further, it does not distinguish between multiple vehicles in shot.
  
  Section::::UK deployment.
   BULLET::::- The Mk 38 Mod 2 weapons system is a remotely operated Mk 38 with an Electronic Optical Sight, Laser Range-Finder, FLIR, a more reliable feeding system, all of which enhance the weapon systems capabilities and accuracy.
   BULLET::::- Mobile VHS video cassette recorder with a remote control unit. VHS is now being replaced with Digital Hard Drive Recorders or DVD recording systems.
  
  BULLET::::- Two colour monitors, one each for front and rear seat occupants.
   BULLET::::- LIDAR speed guns utilize the time of flight of laser pulses to make a series of timestamped measurements of a vehicle's distance from the laser; the data is then used to calculate the vehicle's speed.
  
  BULLET::::- Sensors embedded in the roadway in pairs, for example electromagnetic induction or Piezo-electric strips a set distance apart.
  
  BULLET::::- Infra-red light sensors located perpendicular to the road, e.g. TIRTL
  
  Section::::Methods.:Pacing.
   Section::::Design.:Sensing method.:Acoustic.
  
  Acoustic systems "listen" for either the bullet bow shockwave (the sound either of the projectile or bullet as it passes through the air), the sound of the muzzle blast of the weapon when it fires the projectile, or a combination of both.
  
  Due to their ability to sense at great distances, to sense in a non line-of-sight manner, and the relatively low bandwidth required for transmitting sensor telemetry data, systems deployed for law enforcement, public safety and homeland security in the United States have primarily been based on acoustic techniques.
   Officers in some jurisdictions may also use pacing, particularly where a more convenient radar speed measuring device is not available—a police vehicle's speed is matched to that of a target vehicle, and the calibrated speedometer of the patrol car used to infer the other vehicle's speed.
  
  Section::::Methods.:Cameras.
  
  In recent years many jurisdictions began using cameras to record violators. These devices detect vehicles that are exceeding the speed limit and take photos of these vehicles' license plates. A ticket is then mailed out to the registered owner.
  
  Section::::Methods.:Other.
   Despite the advent of LIDAR speed detection, radar remains more prevalent because of its lower price and the amount of radar equipment already in service. In addition, proper use of LIDAR equipment requires the officer to remain stationary in order to beam a very precise signal.
  
  Section::::Legality.
  
presuppositions: Radar guns need to be pointed at the car in order to work. Cops can only get your speed through a radar gun.
corrections: Radar guns only need to be pointed at the direction of traffic and can work in both directions. Many cops have mounted units on the car that can measure speed.
raw_presuppositions: Radar guns need to be pointed at the car in order to work. Cops can only get your speed through a radar gun.
raw_corrections: Radar guns only need to be pointed at the direction of traffic and can work in both directions. Many cops have mounted units on the car that can measure speed.