When a 6-inch snowstorm hit the Twin Cities during the morning rush hour Jan. 30 and brought traffic to a crawl, Mary Hirsch turned to NexTrip to find out the status of her Metro Transit bus.

The automated real-time and schedule information system told her that her 8:09 a.m. Route 25 bus from St. Louis Park to downtown Minneapolis was running on time.

“I thought that was kind of a miracle,” said Hirsch, a public relations and marketing manager for a Minneapolis law firm. She went to her stop and waited 40 minutes in the wind-whipped snow. “It was lying. How could it lie to me? I’m so sweet. It’s not being invited to my birthday party. I’ve always been happy with it.”

NexTrip works like this: Riders log onto Metro Transit’s web or mobile site. They enter a stop number, or a route number with direction of travel and stop location. A schedule of the next 10 buses serving that stop or route will pop up. If a bus is expected to depart within 20 minutes, NexTrip will list the number of minutes until departure in black. Outside the 20-minute window, NexTrip will list the actual scheduled departure time in red.

Buses are equipped with onboard computers that track their GPS location. As a bus travels along its route and passes time points, it sends information back to Metro Transit’s headquarters. That information is then fed into the NexTrip system and updated every few seconds.

That’s fine and wonderful when all is well in the transportation world. But NexTrip is susceptible to variables that can throw off the information.

If a bus loses communication with the transit center’s headquarters, its arrival and departure time is lost. That can happen when a bus detours from a route in order to bypass spinouts, congestion crashes or other mishaps, or in cases where it’s rerouted around road construction.

Weather is a big factor, too. Buses can only move as fast as traffic and they invariably fall behind schedule. In cases where buses are 20 minutes or more behind, NexTrip doesn’t show an actual departure time, said John Siqveland, a Metro Transit spokesman.

“NexTrip would revert to scheduled times [shown in red] — just like having a pocket schedule,” he said. “Sometimes people get that confused” and think the bus is on time. It does not show how late a bus is running.

Metro Transit riders used NexTrip in record numbers in 2013, accessing it a record 49.2 million times, up from about 30 million in the previous year. Another 2.2 million accessed the system by phone.

Metro Transit recently added NexTrip schedules for the Cedar Avenue bus rapid transit line and the Blue Line light rail on its website. NexTrip schedules also appear on digital signs along Marquette Avenue, at some transit stations and regional park and ride lots.

So when can we expect real-time information on electronic signs at rail platforms instead of messages that say “Please Check Schedules?”

Siqveland said NexTrip information will be added in the coming months at all rail stations along the Blue Line between downtown Minneapolis and the Mall of America in Bloomington and on the Green Line when trains between Minneapolis and St. Paul begin running on June 14. Until then, riders will have to use their phones to access NexTrip or read the static poster-size schedules the rail stations.

Follow news about traffic and commuting at The Drive on startribune.com. Got traffic or transportation questions, or story ideas? E-mail drive@startribune.com, tweet @stribdrive or call Tim Harlow at 612-673-7768.