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Headways and Service Span


Headways and Service Span

Recall that headway refers to the amount of minutes that pass between two successive bus trips and that service span refers to the length of time between the first and last buses of the day. Creative use of varying headways can increase the service span at no additional cost in service hours.

For the sake of this article I will use Valley Metro in Phoenix as an example, but many different transit providers could benefit from this approach. Valley Metro has, like many transit agencies in the United States, been forced to make cutbacks due to lower funding as a result of the recession. As part of this cutback, the city of Phoenix decided that they were only going to pay for bus service until about 10 P.M. weekdays instead of midnight as they had been doing. Although a generally quiet city, Phoenix does have a lot of restaurants and bars that are open late, and reducing the service span has had a negative effect on the ability of service sector workers to get home from work. Instead of cutting the service span, Phoenix could have kept the same service span, at least on busy routes, and still saved money. Here's how.

The last four weekday eastbound trips on Route 29 Thomas Road leave 81st Place at Thomas at the following times: 8:17 P.M., 8:47 P.M., 9:17 P.M., and 9:47 P.M. Imagine instead that the last four weekday eastbound trips left at 8:17 P.M., 9:17 P.M., 10:17 P.M., and 11:17 P.M. With the same number of trips we have extended the service span by 90 minutes merely by changing the evening headway from 30 to 60 minutes.

Of course, making this change assumes that passenger loads on Thomas Road in the evening are not large enough to mandate operating the route every 30 minutes. However, in that case the large ridership would justify a larger service span already. In addition, making this change assumes that there is not a fixed cost that would have to be born as a result of extending the span. For example, if making this change means that the bus garage will also have to close 90 minutes later a supervisor or security guard may have to be paid for an extra 90 minutes of work, and the electricity bill may be higher if the lights and air conditioning are on for an additional 90 minutes. However, these costs are likely to be small and if enough route schedules are changed in this manner the cost associated with any particular route would be very minimal.

Except in rare instances, night service will never attract high enough ridership to be justified based on strict measures of productivity. This fact is why, when service is cut , evening service is usually the first to go. However, night service operates as an essential lifeline service for the transit dependent. Without adequate night service the transit dependent may not be able to access jobs and schooling that could make a difference in their lives. Because of this effect on the transit dependent at least minimal night service should be maintained whenever possible.

However, maintaining night service does not mean that we need to maintain service at daytime levels. In my daily practice I have often sacrificed headway for service span, reasoning that waiting an average of another 15 minutes for the bus (transit professionals often assume an average wait time of one half the headway, so the average wait time for a 30 minute headway would be 15 minutes and the average wait time for a 60 minute headway would be 30 minutes; of course, at these service levels people are generally waiting much less than that because they consult the timetable) is better than waiting for a bus that will not come until the next morning.

The different marginal benefit between decreasing headways and increasing service span has come into play again recently, this time with L.A. Metro's decision to add more evening service on some of their rail lines. If you discount the inherent absurdity of operating a transit line more frequently at 11 PM than you do at 11 AM - the case on the Red and Blue Lines - then there is nothing to distract you from the conclusion that this is obviously a good thing. Indeed, ridership has increased since this policy was put into effect. But consider how much higher ridership would increase if they increased the service span instead. Right now the last trains are around 12:45 AM; suppose the last trains were around 2:45 AM, at least on Friday and Saturday night. Now a large contingent of potential riders - riders who desired to return home after 12:45 AM and therefore could not take advantage of the current service - would be able to use the service. The worst that could happen to someone who missed a train at 11 PM would be to wait another 20 minutes; the worst that could happen to someone who missed a train at 1 AM would be to be dumped out in the cold by security to fend for him or herself. Now tell me again which is more important - frequency or service span?

Overall, I heartily recommend that transit industry professionals examine their system timetables to see if this change would be beneficial to their riders, and transit advocacy groups advocate for this change if the transit system seems reluctant to make it.

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