they are only an approximation of what you really see outside. Those data streams, when you walk out onto the street. People can, and do, trust a highly consistent experience.Īnd yes, I live in London and use CityMapper a lot, and I've also played with those TfL feeds and experienced the real world lag in the data for bus/train arrivals, and bike hire data.
The consistency means that almost everyone is treated to a similar experience, and though it is not highly accurate it is highly consistent. Until that lag can be almost eliminated I think they're doing the right thing by choosing values based on average frequency and average wait time. This experience would vary so greatly in different parts of the city (density of CityMapper users), that it would corrode your trust of the info. If you have a bus frequency of 7 minutes we're actually talking a lag of around 28% of the worst case wait time for that bus. You can estimate a lag of greater than 1 minute, pushing 2 minutes if you were unlucky that another CityMapper user nearby potentially requested the same data just before you and you're now seeing TfL's cache. Then you have the ingestion and processing lag that Citymapper introduce before they get it to you and display it on your screen. You realise that there is some lag in the system, right?Ī small lag in data collection and transmission, a small lag in publishing the data via the feed (this is stated as 30s) and TfL cache the data points for 30s.
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