A speed camera has no single sensor. Depending on the type, the part that reads your speed is a radar, a laser, strips or loops buried in the road, or a pair of number plate cameras that time you over a known distance. This guide takes them one by one, from the National Police Chiefs' Council (NPCC) technical guide for police, Police Scotland, National Highways and the Police National Legal Database. The NPCC guide dates from July 2016; the Vector SR is described from the Police Scotland page.
The sensors at a glance
| Sensor | What it reads | Where the sources place it |
|---|---|---|
| Radar | The speed of a vehicle, by the Doppler effect | Gatso, Vector SR and HADECS cameras |
| Laser | Speed from light pulses reflected by the vehicle | Marked police vans |
| Piezo strips, tubes or loops | When a wheel or a vehicle passes over them, and the time between sensors | Truvelo cameras, Redspeed red-light cameras |
| Number plate cameras (ANPR) | The plate and the time at two or more points | Average speed cameras |
Radar: the common roadside sensor
According to the NPCC guide, an across-the-road radar uses the Doppler effect and shines a beam across the road at a fixed angle. Because the angle is known, the electronics correct for the cosine effect, the way a beam meeting a vehicle at an angle senses less than its true speed, and the speed is read directly. Police Scotland says Gatso cameras operate on radar and Vector SR cameras use tracking radar over several lanes.
On a motorway the same principle sits on a gantry. The NPCC guide describes parabolic radar as lighting up one lane from equipment suspended above it, most commonly in smart motorway applications beside variable limit signs. Police Scotland says the radar of a HADECS system measures the speed of every vehicle passing the gantry.
Laser: the sensor in the mobile van
A laser speedmeter sends a series of light pulses and works out the speed from the movement of the vehicle between them, the NPCC guide says. The guide adds that police equipment uses invisible infra-red light, that retro-reflective parts such as the number plate are the best items to aim at, and that a reading typically takes less than 0.5 seconds. Police Scotland says its vans are clearly marked and measure vehicles both approaching and travelling away.
Sensors in the road: piezo strips, tubes and loops
Ask the Police says a Truvelo camera uses four piezo sensors embedded in the road surface at set distances to measure the speed of passing vehicles. The NPCC guide explains the principle: the pressure of the road wheels on a piezo cable or hollow rubber tube is converted into an electrical pulse that starts and stops a timer, so the device calculates the speed from the time between sensors. The guide says permanent sensors should be checked by the supplier annually or every two years, and that rubber tubes across the carriageway are for temporary mobile systems only.
Red-light cameras use the same idea to find the stop line. Police Scotland says Redspeed loops under the road are triggered when a vehicle passes the stop line after the light has turned red, and the camera then takes two images. The NPCC guide says most approved red-light systems use loops or piezo strips buried just before and after the stop line, and that detectors that become exposed through wear should be reported and the device suspended until repaired. Police Scotland says the Vector SR works in a non-invasive way that removes the need for loops or strips in the road.
Number plate cameras: time instead of speed
An average speed system does not read your speed at any point. National Highways says its cameras calculate the time a vehicle takes to travel a known distance. Each vehicle is recorded by an entry camera and an exit camera using automatic number plate recognition (ANPR), the data is matched, and the average is worked out from the distance and the time. For vehicles that did not exceed the limit, National Highways says the plate data and images are deleted automatically.
The NPCC guide adds that the distance between the cameras, the baseline, is set at the minimum possible driving distance, so the system shows the lowest speed possible between the points. It is measured at least three times, within 1% of the average, and the shortest result is used. The motorway guide covers average speed checks at roadworks in England.
A second check on the first
Police Scotland says a Gatso site has secondary check markings painted on the road. They appear in the photographs and show that the radar speed fits the distance travelled between the two images; only the radar determines the speed. The NPCC guide says such secondary checks should normally fall within 10% of the primary reading, and that portable radar devices generally have no check marks on the road, so the operator becomes the secondary check.
Approval and calibration
Section 20 of the Road Traffic Offenders Act 1988 says a record produced by a prescribed device is not admissible as evidence unless the device is of a type approved by the Secretary of State. GoSafe states that all the devices on its list, from SPECS3 average speed to Vector SR, Gatsometer, Redspeed and laser models, are Home Office type approved. Police Scotland says its cameras are calibrated on a regular basis by a third party. The overview of the four types shows where each sensor fits, and what a flash means covers the light side.
What BlitzBuddy does about it
BlitzBuddy gives no speed camera warnings in the UK, Ireland or any other country outside the five where it warns; your speed, trips and reports from other drivers still work. Camera warnings switch on by themselves on arrival in the Netherlands, Belgium, Germany, France or Türkiye. In Germany, § 23 (1c) StVO forbids drivers to use a speed camera warner while driving; the warnings can be switched off under Profile, Warnings, and the driver is responsible. In France, article R413-15 of the Code de la route punishes holding, carrying or using a device that can detect or disturb the equipment used to record traffic offences with a fifth-class fine and the loss of six points, and its paragraph V, added by a decree of 3 January 2012, extends this to devices or products meant to warn of or inform about the location of that equipment; using these warnings is the driver's own responsibility. Where speed camera warnings are allowed compares the countries.
BlitzBuddy is free, with no ads and no subscription. It is available for iPhone; the Android version is coming soon.
More guides on the same topic are collected under speed camera guides. The cameras that look into the car instead of at the speedometer are in AI cameras for phones and seat belts.
Sources
- Guide for the operational use of speed and red-light offence detection technology, version 1.2.1, National Police Chiefs' Council, July 2016 (pdf)
- Camera Type, Police Scotland Safety Cameras
- Can you tell me about the different types of speed camera most commonly used in the UK?, Ask the Police (Police National Legal Database), on the West Yorkshire Police website, read October 2026
- Average speed enforcement cameras around roadworks, National Highways
- Our cameras, GoSafe
- Road Traffic Offenders Act 1988, section 20, legislation.gov.uk
- Straßenverkehrs-Ordnung, § 23, Federal Ministry of Justice (in German)
- Commentaire de la décision n° 2021-948 QPC, Conseil constitutionnel (in French; quotes article R413-15)
- Permis de conduire : barème des points retirés par infraction, Service-Public (in French, verified 27 February 2026)
Sources read on 16 October 2026.
Frequently asked questions
What sensor does a speed camera use?
It depends on the camera. Radar reads speed at a Gatso, a Vector SR and a motorway gantry, and laser in police vans. Some cameras use sensors in the road, and average speed cameras time a vehicle between number plate cameras.
Are there sensors in the road for speed cameras?
Some, yes. Ask the Police says a Truvelo camera uses four piezo sensors embedded in the road surface, and Police Scotland says Redspeed red-light cameras are triggered by loops under the road. The Vector SR needs no loops or strips.
Who approves speed camera sensors?
Under section 20 of the Road Traffic Offenders Act 1988, a record from a prescribed device is admissible only if the Secretary of State approved that type of device. GoSafe and Police Scotland say their cameras are Home Office type approved.
Does a speed camera sensor measure my average speed?
Only an average speed system does. National Highways says it calculates the time a vehicle takes to travel a known distance between an entry and an exit camera.
