Monday, April 30, 2012

On the Metro Transition to GPS


A user asks:

When will OneBusAway be based on GPS tracking of each Metro bus, so that it will be more accurate and work (at least for locating buses) even when buses have to be re-routed?

Would that it were that simple. We are told that the transition to GPS, already well over half complete, will be over by early Autumn, but it may deliver less than you thought it promised.

Here's the basic idea of how OneBusAway works. There are two parts:

1. About four times a year, OBA gets a schedule from each of the agencies, including Metro. The schedule says "here's where and when all the buses are going to stop from now until the end of the service interval several months from now." For example, it might say- for one of the almost 30,000 KCM trips the schedule describes- that there's going to be a route 16 trip every weekday starting at 7:18 AM- and here's where and when it's going to stop- and this is the ID it will be labeled with.

2. Every day, all day, we get real-time reports of trips- tens of thousands of them. What we get in each report isn't so much "here's where the bus is" but rather "here's how this trip is performing against the scheduled trip." Something like "trip X is running Y seconds off schedule."  Ahead of, behind, or spot on. That's called "schedule deviation."

So- we know where the agency said it would be, and we know what the "schedule deviation" is. We might also know where the bus is, but it's all but irrelevant.

A user is standing on the street at a stop, and she asks OBA: tell me about any trips that are headed to this stop. OBA looks at the schedule and looks at the real-time reports and computes: here are the trips that are supposed to be there soonish, and here's what we know about how those trips are performing. Add it all up, and it's able to give you the display you seen on a smart phone: this trip looks to be five minutes late, we've seen no recent realtime data for this other trip but it's scheduled to be here in eight minutes so we'll show you that, and here are five other trips with this or that status.

How does GPS make any of this better from the old bus-location system? Only this way: the old system delivered less frequent reports of less accurate location data. Therefore, real-time reporting could compute less accurate schedule deviation.

GPS is not a silver bullet. It does make some new stuff possible, such as detecting when a bus is off route, but there is a bunch of design and engineering between where we are and presenting that to a rider in a useful way.  In particular, we are some ways from being able to present the rider with the actual location of a bus that is off-route.

Monday, April 23, 2012

A Metapost

Wayne Watanabe of the King County Department of Transportation and OneBusAway's Mark Hallenbeck have collaborated on Tracking the Bus Tracker Problems, a post at the Seattle Transit Blog that looks at recent problems with real-time bus location reporting for King Country Metro (and digitizing grandpa's classic rock collection). It's a good and geeky read if you skew that way. The commenters are great at cutting us a new one, too.