dave-in-nj:
two tubes are for speed.
I have seen a black box, about 1/2 inch tall and 10" x 10" that was taped to the road (tar tape) in the center of the lane.
inductive counter, one per lane.
takes about one minute to install
then they just cut the tape and leave the remnants.
Multiple tubes for redundancy, not speed. Think of that truck/trailer with 10+ axles with wheels about 8 inches between tires.
Paul