alt Hacker NewsI've seen it work pretty well in a number of places in the form of privately owned minibuses/vans that can rapidly go where the demand is needed.
As an example, all throughout the Eastern Caribbean this system works really well (in my experience better than most centrally planned bus systems in large cities). On any given island you can go to any main road and within a few minutes a minibus will come along. Most of the time if your aren't familiar with the geography, you just tell the conductor where you are trying to get to, and they will make sure that you get off in the right spot to get where you are going or connect to another minibus. Typical cost was ~$2.
Predictability was pretty low, but because of the small size of busses, there were a lot of them roaming around, I don't think I ever waited more than 15 minutes, and that was in very out of the way places.
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I believe this is also how it works in many Mexican cities.
It's really not ideal. Similar systems are common in Central Asia. They make it difficult for travelers to predict journey times, it's unfriendly to tourists, and it's much less accessible to other populations (e.g. the disabled). They also don't scale well to large urban environments or out of the way journeys in my experience.