I am not the OP. Seems like you should be able to set up two break beam detectors: one per set of cones. Process detector output into digital form where you get

I am not the OP. Seems like you should be able to set up two break beam detectors: one per set of cones. Process detector output into digital form where you get a pulse when the detector is broken. Connect them to a microcontroller input and use a fast counter in input capture/compare mode to measure time between pulses. A counter running at 1 MHz (1 us) should do.