Frequently-Asked Questions

Don't buses already have GPS installed? How is this different?

Yes, most buses have Global Positioning Systems (GPS) installed. But our scheme does much more than simply track locations of the buses: Our contribution is a mechanism that automatically adjusts the positions of the buses so that they are equally spaced in time; that is, they arrive with the same headway (time between arrivals). This provides the best service and gets maximum use from each bus.

What is special about this method of headway control?

Its simplicity and practicality. As far as we know, ours is the only scheme in a large academic literature that has actually been tested on a real bus system. Public transit is not an experimental science and managers of transit systems are understandably reluctant to “play” — but our scheme is easy to understand and easy to try.

Is this scheme suitable for routes with long average headways?

Probably not: Research has shown that people want a schedule if headways are longer than 10–12 minutes.

Won't passengers be annoyed if the bus waits at a control point?

We recommend choosing endpoints of the route as control points because there will generally be no on-board passengers. Otherwise, a good choice of control point is any bus stop where passengers can change to other transportation, such as other routes or trains. At such stops passengers generally appreciate the waiting time, which increases their opportunities to make seamless connections.

Where should control points be located?

The ends of an out-and-back route are natural locations for control points, because buses can pause there without delaying in-transit passengers. Other natural locations are wherever the route intersects with other transport modes, such as train or other bus routes.

How many control points should there be?

There is a tradeoff: More control points provide more assertive control; but buses pause at each control point and so more control points mean more idle bus capacity. There is no “right” answer but rather a management decision. Fortunately, it is easy to experiment.

How do you schedule breaks, lunch, end-of-shift, etc. for drivers when there is no bus schedule?

Because the buses have no schedule under our scheme, the drivers have no schedule either. This means that the drivers must be flexible. (Scheduled services have the same problem because it is impossible in practice to keep to a schedule.) This is not so much a technical problem as one of driver expectations and management. This has not caused any significant problems on the Georgia Tech route: the drivers have been very cooperative. It might be a problem on routes that require a very long time to circumnavigate.

If there is no schedule, how is the performance of a driver judged?

Instead of managing by schedule adherence, we manage by wait-time adherence: Our system reports how well each driver follows instructions about when to depart the control points. (Other issues, such as safe driving, courtesy, etc. remain the same.)

Can't a driver game the system by traveling as fast as possible so that they get a longer break at the next control point?

Yes, but this is a management problem and is easily recognized from the recorded logs of bus positions. Anyway, a driver can do this under a scheduled system too.

What do the drivers think of this scheme?

Georgia Tech drivers have been very cooperative and claim to prefer this scheme, because it removes the constant pressure of schedule adherence.

Isn't dynamic headway control an old idea?

Others have suggested various schemes to adjust the headways, but we have seen no other scheme of comparable simplicity and practicality. Indeed many others seem wildly impractical, assuming, for example, perfect knowledge of instantaneous bus locations and velocities and passenger queues and arrival rates.

How is this different from “time frequency scheduling”, such as is used on the DC Circulator?

Time frequency scheduling is the setting of a target headway (rather than a target schedule). It is a step in the right direction but still suffers from the problem that, except in unusual circumstances, the system manager cannot control traffic velocities. It is fine to have a target headway, but the question is whether it can be achieved. It is not enough to show the drivers where the other buses are, and expect them figure out how to fix problems. Our system abandons target schedules or headways as distractions, and focuses on the essential: that headways be as nearly equal as possible. Furthermore, no one has to figure out how to fix an imbalance because headways equilibrate spontaneously. (Incidentally, the DC Circulator uses a schedule; the schedule, however, is not published.)

You have proved a theorem that shows convergence to a common value of headway, but this theorem describes an idealized set of buses. What does it mean for the real world?

Because of inherent and ineradicable variability in traffic velocities, we do not expect real buses to achieve exactly equal headways. Instead, we interpret the theorem to mean that our scheme resists bunching. We expect that bus headways will vary less under our scheme than under a schedule and that gaps in service will not tend to grow, as they do under a schedule.

Your scheme requires a forecast of time until the next bus, but this will never be accurate. What is the effect of inaccuracy on the performance of your scheme?

The mathematics shows that our scheme will continue to resist bunching as long as the forecasts are not both wildly and frequently inaccurate.

How does bus capacity affect this scheme?

Our scheme is independent of bus capacity. But, as for any other system, if buses are regularly full, additional buses are probably needed.