Up until recently, fares for the LRT system in my city were enforced by a random check by transit police, typically by having an officer board trains and check riders' tickets at random times during random days and handing out fines to fare evaders who they caught.

Between around mid-2006 and the end of 2008 I rode the train to work downtown every day. The trains were so crowded during rush hour that it was impossible for Transit police to board trains to check fares, and even outside rush hour, fare checks were very occasional. A monthly pass at the time was around $75 and a fine for fare evasion was around $200 (the first violation was less than $200, and I think it increased until a cap of something like $250 for repeat offenders). I'd worked it out that if I was caught without paying a fare less than once every three months, it would be cheaper to just pay the fine if/when I got caught rather than buy a pass. So I didn't buy a pass and decided to see how long it would take to actually get caught.

The answer was about 18 months. Got a $170 fine. Which I then forgot about and never actually paid. The statute of limitations on that fine has long since expired.