This is a simple simulation of a (public transit) rail station, with stored value media.
A portion of the population has enough value on their cards to simply walk up to the gates and go. Others will not, and have to use slower vending machines. Based on the crowds, how many devices do you need? Based on the population, what the ratio of devices you need?
Note: this simulation refers to patrons 'needing to topup' reloadable tickets, but this could equally represent patrons purchasing one off, or multi-use tickets.