Speed surveys are carried out by highways authorities on a road to measure the average speed of vehicles over time. They do not identify individual vehicles, so are not 'speed traps' as such.
They are often placed in response to complaints from the public or local Parish councils about 'speeding' on a particular road.
Traditionally these surveys have been carried out using a technique known as Archer surveys. A box is attached to a lamppost or signpost alongside the road at ground level, usually by heavy chains and padlocks. Twin rubber tubes are run across the road secured by clips. The tubes are separated by a precise distance. Vehicles running over the strips cause the air pressure in the tubes to change, one after the other. The time difference between the two changes is measured and used to calculate the speed of the vehicle, which is recorded along with the time.
The devices are usually left in place for 7 days to allow an overall picture of vehicle speeds on the road to be obtained, as traffic flow obviously differs at weekends compared to weekdays.
Radar Pods

A radar pod attached to a lamppost in Lincolnshire
A more sophisticated device began to appear in 2007, being first spotted in Lincolnshire†.
This resembles a large black briefcase with multiple padlocks attached high up a lamppost, with the flat side of the briefcase aimed at approaching traffic.
The pod contains a battery operated low power radar device that can not only measure the speed of approaching traffic, but is also able to distinguish between small vehicles, such as cars, and large ones, such as buses and HGVs.
Councils now prefer this type of equipment because: Problems

The information obtained from such devices is fed back to the complainant and used to identify the scale of any problem. The council may subsequently decide to reduce the speed limit, or even increase it (don't hold your breath) or introduce other measures to reduce vehicle speeds.
We are told that more often than not, results show that vehicle speeds are actually lower than people imagine, which comes as no surprise to us given the incessant 'speed kills' propaganda put out by the government.
† Our thanks to Steve Batchelor of Lincolnshire County Council for explaining the mystery briefcases.