Piezo-electric road strips, a known distance apart (1.53 cm from 1 - 3, 2 - 4) are set into the road in front of the camera, and the time between compressions is measured using an impededance converter and two clocks in the camera to give the resulting speed of the vehicle. The system takes a single photograph (front-facing only, two for rear photography) and uses the time from the clocks to calculate the resulting speed. The photograph of the offending vehicle will show its front tyres on the three narrowly spaced white lines across the carriageway (known as a Secondary Speed Check Lines), the middle line is 1.8 m from the last sensor, the first and third are 18 cm either side of the middle line = 10% which are present at all Truvelo installations. This is to identify the offending vehicle when it is travelling alongside a non-offending one.