a calibration device that indicates as a blinking light on the handle.
a perfectly balanced, with a perfect forward thrust and no sideways-whatsoever-inclinations trolley will have the light blinking in the middle of the handle.
a blinking light on the right side of the handle shows that the
trolley will slightly veer off towards the right and thus is suitable for right-handed shoppers.
a blinking light on the left side of the handle shows that the trolley will slightly veer off towards the left and is thus suitable for south-paw customers.
no blinking lights at all indicates that the trolley is not blinking working and should be thrown in the nearest river. no sorry, I didnt mean that. the faulty trolley should be immediately reported to a senior member of staff and the wonky wheels rectified promptly by a qualified blinking trolley technician (holding at the very least a MNIFRWWQ)