In the parking industry there is a phenomenon that's been around since the first parking meters were installed, Piggybacking. This is the process of parking in a recently vacated space which still has yet to expire, granting you free (or reduced parking).
Taking advantage of a paid parking space is most easily accomplished in places where the 'coin-style' parking meters are found, as many of them have a green or red internal flag that can be spotted by the incoming motorist. The surge in Pay and Display and Pay By Space meters has slowed down this process some, and in some cases, people are unhappy at the unintended consequences of the newer technology.
In some cities, people are concerned that the use of Pay By Space meters is allowing their local government of collection parking fees for the same spot, at the same time, from two different drivers. For example, if one driver leaves a spot with time left, and another driver parks there right after, they'll have to pay for parking as there is no way for them to know if the space is already paid for.
Many cities claim that this is an unintentional side effect of the more efficient parking systems. Do you think this is an ethical approach to on-street parking?