Fixed-route scheduling
Introduction
Agencies operating fixed-route buses find the need to make adjustments to route scheduling periodically in response to changing circumstances. Bus transit planners should be aware of the basics of fixed-route scheduling. This topic does not cover development of a new fixed-route service, but rather the tuning or re-tuning of an existing or already planned route.
Different agencies value different qualities in service levels. The minimum or average frequency of service may be set by agency policy. Alternatively, some agencies may focus more on geographic coverage over frequency. Achieving success in both aspects is unlikely due to the high cost of having high frequency and extensive coverage. Priorities can extend further into how aggressive the bus schedule is against traffic congestion, or how relaxed it is by allowing more layover or recovery time along the route.
Basics
The characteristics of the area determine the basic components of service. How frequently will the route run? Where will the bus stops be and what is the distance between them? In small communities, suburban and rural, service may operate once an hour; in major metropolitan areas services may run every ten minutes or even more frequently. Certainly there is a wide range in between. In areas where a route operates on frequent headways, say every 10 minutes or less, passengers more often walk out to the stop without knowing the departure schedule; at most they are likely to only be waiting an average of 5 to 9 minutes for the next bus. When service is less frequent than about every 10 minutes, passengers will have to rely more heavily on the published schedule. In these cases it becomes even more critical that the route timetable is appropriate for operating conditions such as traffic levels at different times of day.
While some agencies may calculate average time between every stop, it is more common for agencies to designate scheduled "time points" along a route. A time point would be a major stop by which the driver should not pass earlier than what is published. The precise time for stops in between is unpublished. Passengers read the schedule and if they are waiting at a stop in between can reasonably estimate when the bus will come based on the time points on either side of their stop. This can simplify published information and also allow flexibility on stretches of the route with a high variability of speed.
To establish a basic schedule, it is best to set time points that are evenly spaced along the duration of the route. A reasonable guideline would be a time point every five to ten minutes apart. Agencies often set time points using major and recognizable landmarks or intersections. Calculate the average free-flow (no traffic delay) time between each time point.
Putting it all together
Add all of the time between time points in free-flow travel. The schedule should be padded with what is known as "passenger-stop" delay time. This is the average time the bus is delayed from moving to board and alight passengers. This amount of time varies between agencies. It can be affected by many factors, such as:
- How fares are handled (cash only systems tend to experience more delay than those with electronic fare media)
- Whether agency policy is to wait for passengers to be seated before departing
- Whether the bus stop requires the bus to reenter heavy traffic
- How many average boardings are estimated at certain stops
Delay must also be calculated for other factors, especially traffic congestion and traffic control such as signals, stop signs, and left turns across traffic. These can vary tremendously even on different lengths of the same road.
Finally, "recovery" or layover time should be added to the route. This serves as an insurance policy against abnormal delay on one trip significantly impacting subsequent trips. It is also essential for operations staff; driving a bus for any length of time is wearing on an operator. Bus drivers need time to stretch, use the bathroom, eat, etc. Routes with too little recovery time are stressful for the driver, and in turn the passengers, especially if the route suffers from poor on-time performance.
Recovery time
A good rule of thumb is to include an additional 10% of the trip-time for recovery. Layovers can be scheduled where it is convenient for the service. It may seem most straightforward to add a few extra minutes of layover at the end of the route. However, in some cases the travel time between two time points in the middle, for example, may be so variable at different times of day that some layover time between the two would be appropriate. On the other hand, too much scheduled delay in the middle of the route discourages choice riders from using the service as it appears to be inefficient. Operator needs and safety must also be considered; layovers should be in a location that operators are safe and feel comfortable, as well as have access to restrooms.
Planners may sometimes be tempted to cut recovery time in order to keep a route to an even schedule. For example, imagine that a trip including travel time and delay time is 58 minutes long. 10% recovery time would be at least an additional 5 minutes, but that would put the route over 60 minutes. If the route is 60 minutes long, it can be operated by two buses on exactly the same schedule every hour. However, this makes the recovery time only 2 minutes. There are trade-offs a planner must consider. With only 2 minutes of recovery time, the margin of error for dealing with unplanned delay such as a traffic accident is very slim. On the other hand, schedules which vary from hour to hour are confusing for passengers (unless the service is so frequent that the published schedule is nearly irrelevant).
Adjusting for delay
Because planners cannot control every factor on the road delaying a bus route, they must instead focus on adjusting factors that are within the agency's control. In the example above, a planner faces the choice of making a very tight schedule with little recovery time, or creating an inconsistent schedule that shifts over the course of the day. To alleviate this issue, service could be cut somewhere to bring the route time down. However, there may be other strategies that are also appropriate.
Scheduling a route should, as much as possible, take a holistic view of policies and operating procedure. Agencies should consider the delay factors noted above and how they can manage them to reduce travel time. For example, encouraging passengers to use fare media and passes that are more efficient for boarding than fumbling for exact change can speed up boarding time. Strong operator training programs can facilitate tighter operating times without sacrificing safety and comfort. Some agencies may be able to accommodate all-door boarding (typically using electronic fare media).
In the bigger picture, agencies should plan and advocate for bus stop construction that allows curb-height boarding and travel lanes that are exclusive to buses. Some agencies have been able to implement transit signal priority to cut down on route travel time.
On the other hand, routes that include a flexible component such as request stops or route deviations need to be scheduled allowing enough time for the deviation.
Additional Reading
Walker, Jarrett. 2012. Human Transit: How Clearer Thinking About Public Transit Can Enrich Our Communities and Our Lives. Washington, DC: Island Press.