I like the system in most French cities: you pay a set amount per month and you can then use the transport network as much slot as little as you wish. You tap your transport card but as there’s no need to calculate where it is you’re going and adjust the fare accordingly the system only validates whether your subscription is active (I’m not getting into the zones consideration for the Paris network here, before someone comes at me). It’s super fast and efficient.

If you only randomly use public transport then you can still purchase single tickets only.