It depends on the specific transit system. In my experience in NYC, the subway data is extremely accurate in terms of the minimum time until the next train. Th

It depends on the specific transit system. In my experience in NYC, the subway data is extremely accurate in terms of the minimum time until the next train. The subway virtually never arrives "early". Buses seem to have a problem where their location transmission sometimes fails for a few minutes. The system always assumes the bus is still stuck at its last reported location rather than moving. That's why you get a bus arriving when the feed says it's 4 minutes away. A good rule of thumb is that if you see the minutes away change, e.g. from 7 to 6 minutes, it's accurate. If it's not changing (e.g. just sits at 7), it might be because it's genuinely stuck in traffic, or because it's stopped transmitting. (Which explains the "delayed by" situation you describe.)