Average speed cameras: how they actually work

Average speed enforcement has become the default on motorway roadworks and on a growing number of permanent stretches. It behaves differently from a fixed camera, and most of what drivers believe about it is folklore.

How it works

Cameras at two or more points read your number plate and timestamp it. Divide the distance between them by the elapsed time and you have your average speed over that section.

There is no flash, there is nothing to see, and slowing down for the camera you can see achieves nothing because your speed at any one point is not what is being measured.

The myths

"Change lanes and it can't track you." It can. Modern systems read plates across all lanes and match entry to exit regardless of which lane you used.

"There's a grace period so you can make it up." There is generally a small tolerance, as with any enforcement, but it is not a licence to speed for half the section and crawl the rest. The average is the average.

"They're switched off at night." Assume they are not.

"They're only in roadworks." Increasingly not — several permanent stretches use them, and they are common on rural A roads with poor safety records.

Fixed versus average speed cameras

Why roadworks use them

Two reasons that are worth understanding rather than resenting.

Workforce safety. People are working feet from live traffic, often behind nothing more than a cone line. The 50 mph limit exists because of them.

Throughput. Counter-intuitively, a uniform lower speed can move more vehicles through a constrained section than a higher speed with stop-start bunching. Smooth flow beats fast flow when capacity is the constraint.

For anyone driving for work

Two practical points.

Points are worse for people who drive for a living, because exposure is higher and because a licence may be a condition of the job. A single average-speed section is a very easy way to collect three points on a stretch you have driven safely for twenty minutes.

And roadwork sections are long. Setting cruise control at the limit is the whole answer — it removes the drift that catches people, which is almost never deliberate speeding but a gradual creep back up after the first mile.

The journey-time cost

If a route regularly puts you through a long 50 limit, that is a real and repeated cost to your working day. Worth knowing precisely rather than approximately: a year of recorded journeys tells you what a corridor actually costs in time, which is a better basis for changing route or timing than a general sense that it is bad.

Sources

• GOV.UK — Speed limits
• The Highway Code — Motorways (rules 253 to 273)
• GOV.UK — Penalty points (endorsements)
• National Highways

General road safety information, not legal advice.