Behold the “reference wheel.” It’s simply a wheel mounted to a block of wood so the wheel can stand vertically.
The Caltrans suggested “reference wheel” to use for traffic signal calibration is a 20″ BMX wheel. Below is Bob Sutterfield‘s “AB1581 loop detector test rig.” He built it from a castoff 20″ wheel and a scrap piece of wood. The traffic engineer simply places this wheel vertically over the detector in the pavement, adjusts the sensitivity controls, then tosses it in the back of his work truck with his safety cones and tools.
The image is Bob Sutterfield’s.