The LTi 20-20 Speedscope is the most common mobile speed measuring system operated by police forces and, particularly, so-called "safety camera" partnerships.
The system consists of two main components:
Below you can see what the video replay of the LTi 20-20 system looks like, and the information shown on it.
Click here to view the image full-size
The following information is contained in these fields (this letter from ACPO may help you to understand it better):
The time and date when this frame was recorded.
The PAL video records 25 frames per second, each made up of two interleaved scans. The frame count is the number of these interleaved "frames" since the start of the second shown in the time field.
"NR" just stands for "New Record".
A number of error conditions can occur when using the equipment, and these are recorded on the video. Some codes that are relevant to the "target" are:
The operator can also manually set a speed threshold. When he does this, a "*" marker appears in the frame if a speed reading is obtained that's higher than the preset threshold, as shown in this frame.
Case File example
It's been too long since the laser measured the speed for the reading to be considered valid
Unstable reading due to poor aim or panning off of target
Not surprisingly, the recorded speed is above the pre-set threshold! The device has also timed out since doing its speed calculation.
You should view the video in real time to decide for yourself how accurate this speed "reading" is!
Loss of target due to beam interruption or target going out of range