Back in the 1970's, when I lived in Ohio, I used to listed to the then VHF repeaters of the Turnpike. One funny thing was at the end of the day, each gate reported a number. I assume this was the money intake. They used a verbal code that would change monthly. The first month, A=1, B=2, C=3, D=4, E=5, G=6, H=7, I=8, J=9, and K=0 (they did not use F, P, and S). The next month they started where they left off: L=1, M=2, O=3, Q=4, R=5, etc. When they got to the end of the alphabet, they went back to the beginning. This was figured out by watching over several months, first noting during a month when say J... became A.... that J was probably 9 and A was probably 1 with 9... getting larger to 1.... (requiring another digit). Fun but useless. (During the same time, the FCC DF network used HF repeaters and coded their Baudot with simple bit inversion that changed daily in just as simple of the way. I added $2 of parts to my Baudot RTTY decoder andI would automatically set up by setting DIP switches for the next day when it came and would know where ever their DF found a fix. If I lost track, I had made a table for the inversions of Downspace-Carriage_Return-Line_Feed and would instantly know which bits were being inverted. I knew where they DFed pirate broadcasts, drug dealer etc. Many years ago, they abandoned the HF DF repeater network to satellite, as told to me by an FCC person who visited me on another matter, and today that system is remoted.)