Home     Links     Contact Us     Bookmark  
   Homepage      News      Legal Forum      Dictionary  
Home : Legal Forum : Cars & Transportation Safety

How do police detect speed?
Find answers to your legal question.

How do police detect speed?

How do police know that im over speeding and pull me over, you know its hard to tell how fast the car is moving especially when you all driving, anybody knows?


Usualy they use a speed gun when parked. That bounces a laser beam at the car and bounces it back to find the speed when the lasor breaks. When they are behind you and pull you over for speeding, they usualy match your speed and just look at their speedometer. That tells them what speed your going because their going that speed as well. For going the oppisite direction of you on a road way, i have no clue. I guess they compare you to other cars or guess your speeding.

Up here in NY they can now tell how fast your going even while passing eachother,on coming he's in his lane and you're in your lane and pass eachother,so if you live here and you see a cop oncoming be sure your doing the speed limit,i have no idea how they came up with that one,mabey in your state to,cause eventually it's going to be nation wide.

Pulse Doppler Radar.

To Learn more see these links

Modern police cars are equipped with a wide range of devices to detect the speed of other vehicles. Some are used at the roadside - laser/radar - whilst others allow accurate measurements of the speed of vehicles on the move

VASCAR - Visual Average Speed Computer and Recorder
Early methods of speed detection relied on the use of a stopwatch to time cars passing between fixed points. Vascar is essentially a stopwatch - but with the addition of a distance data feed from the moving police car can measure speeds on the move - and there is no need for the police car to be travelling at the same speed as the speeding car, behind or even in the same direction!
Vascar units have two sets of switches - to allow the pursuing police officer to measure the time that a car takes to travel between markers and the distance between the same markers as covered by the moving police car.
As the speeding car passes the first marker the timer is switched on. As the police car passes the same marker the distance recorder is switched on. As the car passes the second marker the timer is switched off, then the distance recorder is switched off as the police car passes the same point. The police now have accurate measurements of both distance and time and the speed of the vehicle can be calculated.

Radar and Laser detection - portable and hand-held devices
Radar speed detectors (radio detection and ranging) transmit a radio frequency signal which is reflected from the target vehicles. The systems operate in the microwave band transmitting waves at around 0.3 to 0.1m wavelength. The speed detection can operate in two ways:

Pulse mode: a series of pulses is transmitted at a fixed time interval. The system measures the time between the transmitted wave leaving the radar unit and wave reflected from the target arriving back at the base system. As the speed of the radar waves is known (and fixed), the time taken can be used to calculate (automatically) the distance to the target. If the distance is measured over a number of pulses (transmitted at fixed time intervals), then the speed of the vehicle can be measured.

Doppler mode: radio waves are reflected back from the target with a slight change in frequency - and the amount of the change is proportional to the speed of the vehicle. By comparing the frequency of the outgoing wave with that received, the speed can be calculated.
In both modes, the speed is calculated automatically and is usually displayed on an small screen at the rear of the gun. An police officer can point the gun at an approaching car, get a reading of the speed and flag the car down if necessary.

The speed measurement is most accurate when the gun is pointed directly at the approaching vehicle. The radio wave transmissions of the radar devices can be detected by other receivers - some drivers use small 'snooper' devices to warn them of radar transmissions (not always with a great deal of success!). Radar systems are often used as the speed detection and triggering mechanisms for roadside cameras.

Laser detection and range device

Laser based speed detectors are the latest device available to the traffic divisions. Working in a similar manner to radar devices, they rapidly measure the distance from the laser gun to an approaching car - and repeat the measurement many times in just a few seconds to obtain a fast and accurate detection of speed. Again, the calculations are performed automatically and the resulting speed is displayed on a small screen.

In use, a police officer will aim the laser at an approaching car and press the trigger to activate the device. A speed measurement will be indicated on the display and, if this is above the agreed level for that road, the officer will step out and flag the car down. The offending driver can then be shown the speed recorded on the screen, together with the distance at which the speed was monitored.

The laser detector transmits light in the invisible infra-red part of the electromagnetic spectrum. A driver targetted by such a device will be unaware that it is in use - and laser speed detectors cannot be detected by 'snooper' systems. The laser beam width is very narrow, much narrower than the radar beam width, so individual cars can be picked out at some distance from the detector.

A police aircraft will circle overhead at an altitude as not to be seen by traffic and uses the Vascar system

When trying to get a speed reading of the motorcycle going 80 mph passing the SUV going 65 mph, a lidar operator simply aims at the motorcycle. Lidar units don't have or need fastest mode because lidar targets only the vehicle the officer chooses. Many vehicles can simultaneously travel through the wide radar beam, which can bring into question which vehicle produced the displayed reading. (A defendant may bring up this argument in court, but an experienced, well-trained officer should be able to testify as to their tracking history, i.e., how they knew the defendant's vehicle was producing the displayed speed reading and, subsequently, that the ticket is valid.)

Radar units use a Doppler tone to help the officer confirm the speed displayed by the unit is actually the intended target vehicle's speed. Lidar units do not provide a Doppler tone because they display the speed of only the vehicle the officer selects. Lidar does have a target-acquisition tone that remains constant regardless of the speed of the target vehicle (one particular unit simulates a Doppler tone that does correspond to the speed of the vehicle). The tone confirms that the officer has, in fact, obtained a valid speed reading.

Lidar has some unique capabilities as well. For example, when coupled with a mapping program, the range mode on a lidar device provides quick, easy and professional documentation of traffic collisions and crime scenes. If your department does not have the budget for expensive (and bulky) surveying equipment, a lidar with mapping capabilities may be just the ticket. Lidar can instantly give an officer an accurate measurement of objects several thousand feet away. At the scene of a collision, a traffic officer can quickly (and safely) measure an intersection without leaving the curb. In the case of a crime scene, indoors or out, a lidar's mapping software can produce detailed diagrams without touching or moving evidence.

A radar device emits a concentrated radio wave and listens for any echo. If there is an object in the path of the radio wave, it will reflect some of the electromagnetic energy, and the radio wave will bounce back to the radar device. Radio waves move through the air at a constant speed (the speed of light), so the radar device can calculate how far away the object is based on how long it takes the radio signal to return.

 Enter Your Message or Comment

User Name:  
User Email:   
Post a comment:

Legal Discussion Forum

Copyright (c) 2009-2013 Wiki Law 3k Wednesday, April 23, 2014 - Trusted legal information for you.
Archive: Forum  |  Forum  |  Forum  |  Links