Metro Transit knows its NexTrip app isn’t always accurate in real-time tracking of buses and trains — a frustration for people trying to catch a ride. Here’s a look at how it works and a plan to improve it.

How does NexTrip work?

Every Metro Transit train and bus has an onboard computer that records its GPS location. As vehicles move, NexTrip tracks their location in relation to specific time-points and uses an algorithm to predict expected arrival times. It updates every eight seconds.

Why doesn’t the bus always come when NexTrip says it will?

The algorithm doesn’t always have everything it needs. Say in normal conditions it takes a Route 32 bus five minutes to get from Penn and Lowry avenues N. to its next check-in point at Fremont Avenue. NexTrip tells riders waiting at Fremont their bus will come in five minutes. If there is a fire, a crash or a storm causing travel delays, that is not factored in — and NexTrip doesn’t reflect that delay because the bus has not reached the next check-in point.

How is Metro Transit fixing that?

Metro Transit will work with Cambridge Systematics to update NexTrip and make the algorithm smarter. If the retooled algorithm notices a pattern of buses that aren’t passing check-in points on schedule, it will adjust NexTrip to reflect those delays. It still won’t be perfect, but the company says it can improve accuracy from 65% to 80%.