Explain how the development of car$-$sharing services in major urban areas, such as ZipCar, punctuated the equilibrium for well$-$established car$-$rental companies like Hertz and Avis. Note: With ZipCar, people pay a monthly fee to join the service; they can then reserve cars by the hour or by the day, and pick them up in convenient locations. The process is much easier, with much less hassle, than going through a traditional car$-$rental company.