Average speed cameras: how do they work?

As modern cars get faster and more powerful, average speed cameras are becoming more common. With the potential to get up to high speeds more easily, it’s more important than ever to pay attention to the prevailing speed limit. 

Average speed check technology also helps maintain traffic flow; steady speeds are proven to reduce congestion, and therefore have the potential to make our roads much safer. But how do average speed cameras work, and where might you find them?

What is a smart motorway?

Average speed cameras differ from the fixed Gatso-style cameras (named after Maurice Gatsonides, the Dutch rally-driving inventor) that have become a common sight on UK roads over the past two or three decades. These tend to be placed in locations where dropping drivers’ speed is especially important, such as outside schools and at accident hotspots. 

The introduction of these cameras has created a measurable increase in the number of penalty points drivers have received over the past two decades, but they only capture a snapshot of a driver’s speed at a particular time and place. Once a motorist has cleared the speed camera, they may be tempted to resume driving above the posted limit.

Average speed cameras are arguably more effective at enforcing speed limits. They use a series of cameras that are linked together and monitor a driver’s speed over a longer stretch of road, so there really is no escape – you have no choice but to comply with the speed limit in these areas or risk getting caught breaking the law. Doing so will incur a fine and penalty points on your licence, or see you having to attend a speed awareness course.

Where can you find average speed cameras?

Officially, average speed cameras are located wherever traffic speed has been identified as raising safety concerns. Common locations are along busy stretches of A-road, be they single or dual-carriageways, and temporarily in roadworks zones, especially on motorways.

It is becoming increasingly popular to use an average-speed camera to enforce lower limits, such as in villages. An average-speed camera placed at either end of the village leaves traffic no choice but to abide by the lower limit. These are usually prominently positioned with advanced warnings and are highly likely to show on speed camera locator apps.

Variable speed limits do not typically use average speed cameras – 'smart motorways' such as sections of the M25, M6 and M62 instead use fixed-point cameras named Hadecs 3. Not necessarily painted in the high-visibility yellow of other speed cameras, these are typically mounted at the nearside of grey motorway gantries, although locations can vary. They operate whenever a temporary speed limit restriction sign is illuminated, but can also be used to enforce the national 70mph motorway speed limit.

Are all average speed cameras the same?

Among the most common types of average speed camera are SPECS cameras. These are usually mounted on roadside gantries at regular intervals of more than 200 metres, although the latest SPECS 3 cameras need only be 75 metres apart. In terms of appearance, they closely resemble security surveillance cameras such as those found in urban public areas and are mounted in yellow plastic housings. They're effective day and night, with infrared illumination sufficient to clearly capture number plates once it gets dark.

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As well as being installed in permanent locations, SPECS cameras are also used to enforce temporary speed limits, for example during major roadworks, where lower limits are introduced for the safety of traffic and the construction workforce.

The latest VECTOR cameras look similar but are more versatile – as well as speed limits, they are used to monitor for offences committed at bus lanes, level crossings and traffic lights, and to enforce congestion-charging schemes. They can be mounted on their own gantries or attached to streetlamps or other tall urban street furniture. They can be either forward or rearward facing and one camera can monitor two lanes of traffic.

Both camera types incorporate automatic number plate recognition (ANPR), which allows an offending car to be identified and linked to its registered keeper.

How do average speed cameras measure your speed?

Any average speed camera system requires at least two cameras linked together, but there's no limit to the number of cameras that can be combined in a system, nor is there any specific limit to how long an average speed camera network can be.

When a driver passes through the first camera in a linked system, ANPR technology takes an image of the car’s number plate. As the car goes past subsequent cameras, they measure the time taken for the car to reach them, calculating the average speed of the car. If it is found to have exceeded the speed limit, the car’s details are automatically submitted to a prosecution database.

Individual cameras don't have the facility to measure vehicle speed – a car must pass a second camera for its average speed to be calculated. In a sequence of multiple cameras, it is at the discretion of the local enforcement agency as to whether cameras work in pairs or in larger groups.

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Will I know if I've been recorded by a camera?

If you have passed an average speed camera, you will have been recorded by it. Only by ensuring that your average speed is below the legal limit can you ensure that a prosecution notice won't land on your doormat.

Whereas static single-point cameras use a powerful ‘double flash’ to illuminate car number plates when an offence is committed, SPECS and VECTOR cameras use invisible infrared illumination in failing light; there are few weather conditions that render number plates impossible to capture.

Do average speed cameras flash?

The tell-tale flash when a motorist triggers a standard Gatso speed camera is absent from linked average speed camera networks. Average speed cameras instead use infrared technology to detect a car’s number plate – even at night.

Is there an average speed camera tolerance?

It’s a myth that speed cameras allow a 10% plus 2mph leeway when driving over the speed limit. While the National Police Chiefs’ Council (NPCC) recommends this buffer, it’s up to each operator’s discretion as to whether they allow any leeway at all. Legally you could get a ticket for going even 1mph over the speed limit.

What is cruise control and adaptive cruise control?

Exceeding the speed limit is against the law and a network of average-speed cameras that measures the time it takes a car to complete a set journey will provide all evidence required to secure a speeding conviction.

However, not every instance of the speed limit being exceeded results in prosecution. Individual police forces reserve the right to exercise discretion in determining whether further action is appropriate. There's no hard and fast rule, though, and there might be times when a zero-tolerance policy applies. Similarly, drivers may be prosecuted in different ways, depending on the offence and their history. In some cases, for example, they may be offered the opportunity to attend a speed awareness course in place of receiving points on their licence.

It's very unlikely that an appeal based on mitigating circumstances will quash a prosecution. It's always up to the motorist to drive safely and at an appropriate speed. Any instance of speeding might be seen as the result of a decision made by the driver and could even be treated as reckless or dangerous driving as well as simply speeding.

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