Sunday, September 07, 2003

Seattle’s Free Buses - a Constraint-oriented Explanation

Seattle has a ride-free bus service within a limited zone around downtown. It operates from 6am until 7pm every day. People have speculated that it is a county government perk for tourists, or perhaps for office workers, or maybe the politicians just want to be nice to the voters! I tend to think this is unlikely. Perhaps systems thinking and the Theory of Constraints offer a better explanation.

Public transport systems can become virtuous or vicious cycles - the more they get used the more provision of service, and the more available service, the more usage. Equally, the corollary is true, the less usage, the less service is provided which leads to less usage. There is a subtle tipping point between the two and a stable system seems hard to achieve. If service is unreliable and infrequent then the paying public will go elsewhere and usage will fall. This will lead the supplier to reduce service and soon there is no benefit from a public transport system at all.

So it is desirable to have both a timely and frequent service. If buses are to run on-time then uncertainty surrounding the scheduling of the timetable must be reduced. Processing large batches of passengers at a few stops in downtown has more irregularity than processing smaller batches at the many stops in the urban neighborhoods and suburbs. However, if passengers don’t have to mess around finding change and drivers don’t have to collect fares then the irregularity associated with large numbers of people entering or leaving a bus is reduced. Making the downtown area ride free improves the likelihood that the timetable can be met. It helps to keep buses moving and to keep traffic flowing in downtown. The systemic effect encourages usage through improved quality of service.

All that was required to make this happen was a change to policy constraints - the rider must always pay and that payment must always be made on entry. As soon as these policies were waived in favor of a policy where riders going downtown, pay on entry and those going out-of-town pay on exit, then the problem of irregular flow is reduced and buses are more likely to run on time. The side-effect and added passenger benefit is the emergence of a ride-free zone. The metro transit company is trading off the localized revenue lost in downtown against the gain from increased usage of the system by longer distance travelers.

Often the throughput or effectiveness of a system element can be limited by a policy constraint. The policy may be having a negative effect on the whole system. Eliminating the policy may produce both a local improvement and a global improvement - as is the case with the King County Metro Ride-Free Zone - buses run on-time but even better more overall use is made of the bus service.