In the city where I live, we have both paper and card bus tickets – the paper ones are one-time, and a card can be reloaded with more tickets or a weekly/monthly pass. However, when you get on the bus, you have to go through the front door (well, you’re supposed to), and validate your ticket or pass. This significantly slows down the process of loading and unloading people, as the incoming people all need to fit in the front door. (It gets even worse when someone is attempting to buy a ticket from the bus driver, mostly blocking the entrance and delaying the bus further.)
We were in Prague last week, and Budapest this week. Both cities approach it differently. There are validation machines at the entrances to the bus/tram/metro, but they’re near all of the entrances. You enter any of the doors and then validate, if necessary. If you have a daily/weekly/monthly pass, you don’t need to validate at all.
I was amazed how much faster stops were using this system. Of course, I’m sure there are those who free-ride and don’t pay for a ticket, but they have enforcement officers for that, who are sitting at a random stop checking tickets. A stop is drastically quicker when you can go in any door and can exit from any door.
This increases the capacity of the system, as well, as it requires less busses to cover the same time period. If a bus can shorten the time needed to do one lap, it can get more laps in each day. More laps = more capacity (or less busses needed to cover the same capacity).
I wonder how cities choose which method they use for validating tickets and passes? It seems like there are some best-practices, for sure, although there are a lot of requirements (public/private ownership, affordability, existing systems, etc).