Re-examining Nextbus

There’s been a lot of anecdotal complaints about the accuracy of the Nextbus system we use to display Streetcar arrivals. While I find it useful for the way I use the system (I walk until a Streetcar catches up to me), I set out to gather some data. So for a couple of weeks I had the same routine that pulls data from Nextbus for the Transit Surfer do some data capture. I picked one stop (NW 10th and Couch) and had it record predictions once per minute.

This is a limited check, there’s no independent verification of when the vehicles arrive, all we can do with the data is compare how the predictions track over the course of a trip. So that’s what I did. We assume that the final prediction before the arrival is correct and use it to establish the time of arrival. We can then compare the prior predictions against the time they were recorded to see how accurate they are. Hint: there are some problems.

good

A trip with “good” predictions looks like a declining 45% line, the actual time to arrival and the prediction are equal.

pessimistic

Perhaps the worst case is a pessimistic prediction – the Streetcar has come and gone before the predicted time!

optimistic

The much more common case is an optimistic prediction. This is pretty common in the afternoons and in fact around 5pm, the system appears to regularly be optimistic by several minutes.


My fundamental conclusion is that Nextbus’ calibration algorithm is not dealing well with the time of day variations in Streetcar’s rate of progress.

I’ve shared the data with Streetcar management and I suspect it will be a topic of conversation with Nextbus. But I wonder if we want to re-evaluate our strategy? Would it be better to display the location of the next vehicle (e.g., 10th and Yamhill) rather than a time prediction? Would that be more informative or more useful in making choices?

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