Cyclists submit videos to the Police's Operation SNAP system. A significant number of these videos concern 'close passes'. Typically the cameras used have a very wide field of view which makes objects appear to be further away than they really are.
These pages describe the use of ultrasound sensors to measure passing distances much more accurately.
Ultrasound sensors operate by transmitting pulses of ultrasound (high frequency sound in-audible to humans). If the sound pulse encounters a solid object a portion is reflected back to the transmitter. The time taken between transmitting and receiving pulses (multiplied by the speed of sound) indicates the distance of the object times two (the sound has to go there and back)
The system includes a GPS receiver which allows both the time and location of incidents to be recorded accurately. By combining these measurements with contemporaneous video recordings compelling evidence can be provided to support SNAP submissions.
The system produces a variety of outputs, the principle ones are
The video below shows a car passing a cyclist equipped with the ultrasound sensor. The system overlays the actual measurement on the video.
The map extract below shows the location of the above incident. Coloured dots are used to indicate how close the pass was. Red means between 0 and 50 cms, orange between 50 and 100 cms, and green between 100 and 150 cms.
The actual measurements are also available in tabular form. The table below shows measurements taken over a 3 second period. 360 cms is the maximum distance that the sensor can measure. When the red car passes this drops to values in the range 49 - 60 cms (shown in red). In the just over one second period the car takes to overtake it appears to be getting closer to the cyclist. This is confirmed in the video.
