LIDAR speed gun
Encyclopedia
A LIDAR speed gun is a device used by the police for speed limit enforcement
which uses LIDAR
to detect the speed of a vehicle. Unlike Radar speed guns which rely on doppler shifts
to measure the speed of a vehicle these devices allow a police officer to measure the speed of an individual vehicle within a stream of traffic.
The use of many reflections and an averaging technique in the speed measurement process increase the integrity of the speed reading. Vehicles are usually equipped with a vertically oriented registration plate
that, when illuminated, causes a high integrity reflection to be returned to the LIDAR - despite the shape of the vehicle.
In locations that do not require that a front or rear registration plate is fitted, headlamps and rear-reflectors provide almost ideal retro-reflective
surfaces overcoming the reflections from uneven or non-compliant reflective surfaces thereby eliminating “sweep” error. It is these mechanisms which cause concern that LIDAR is somehow unreliable.
program Inside Out
demonstrated how the LIDAR speed gun most commonly used in the UK, the LTI 20.20 could create exaggerated reading. Errors came from two sources. 'Sweep errors' were as a result of the camera not measuring the distance to a fixed point on the vehicle but instead being 'swept' along the side of the vehicle. This was demonstrated by sweeping the target along a wall which was recorded as moving at 58 mph. Another way of achieving a bogus reading was where the laser reflected off a wing mirror, hit a stationary reflective object and then returned reflecting off the mirror a second time.
Speed limit enforcement
Speed limit enforcement is the action taken by appropriately empowered authorities to check that road vehicles are complying with the speed limit in force on roads and highways. Methods used include roadside speed traps set up and operated by the police and automated roadside 'speed camera'...
which uses LIDAR
LIDAR
LIDAR is an optical remote sensing technology that can measure the distance to, or other properties of a target by illuminating the target with light, often using pulses from a laser...
to detect the speed of a vehicle. Unlike Radar speed guns which rely on doppler shifts
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
to measure the speed of a vehicle these devices allow a police officer to measure the speed of an individual vehicle within a stream of traffic.
How it works
LIDAR relies on the principle of time-of-flight of a laser beam to calculate the speed of the target object. Devices typically send out a stream of approximately 100 pulses over the span of three-tenths of a second. The reflected signals are then processed using statistical algorithm to pick which reflected signals to retain.The use of many reflections and an averaging technique in the speed measurement process increase the integrity of the speed reading. Vehicles are usually equipped with a vertically oriented registration plate
Vehicle registration plate
A vehicle registration plate is a metal or plastic plate attached to a motor vehicle or trailer for official identification purposes. The registration identifier is a numeric or alphanumeric code that uniquely identifies the vehicle within the issuing region's database...
that, when illuminated, causes a high integrity reflection to be returned to the LIDAR - despite the shape of the vehicle.
In locations that do not require that a front or rear registration plate is fitted, headlamps and rear-reflectors provide almost ideal retro-reflective
Retroreflector
A retroreflector is a device or surface that reflects light back to its source with a minimum scattering of light. An electromagnetic wave front is reflected back along a vector that is parallel to but opposite in direction from the wave's source. The device or surface's angle of incidence is...
surfaces overcoming the reflections from uneven or non-compliant reflective surfaces thereby eliminating “sweep” error. It is these mechanisms which cause concern that LIDAR is somehow unreliable.
Erroneous readings
In 2005 a BBCBBC
The British Broadcasting Corporation is a British public service broadcaster. Its headquarters is at Broadcasting House in the City of Westminster, London. It is the largest broadcaster in the world, with about 23,000 staff...
program Inside Out
Inside Out (BBC TV series)
Inside Out is the brand name for a number of regional television programmes in England broadcast on BBC One. Each series, made by a BBC region, focuses on stories from the local area...
demonstrated how the LIDAR speed gun most commonly used in the UK, the LTI 20.20 could create exaggerated reading. Errors came from two sources. 'Sweep errors' were as a result of the camera not measuring the distance to a fixed point on the vehicle but instead being 'swept' along the side of the vehicle. This was demonstrated by sweeping the target along a wall which was recorded as moving at 58 mph. Another way of achieving a bogus reading was where the laser reflected off a wing mirror, hit a stationary reflective object and then returned reflecting off the mirror a second time.