The flexible-route bus concept would blend the regularity of traditional bus service with the range of a taxi

Earlier this week we looked at what a large city can do to increase the efficiency of its bus service in a high-density corridor. In Manhattan, bus stops are so crowded that transit officials must find ways for passengers to board more quickly. That's a problem, of course, but in the grand scheme of transit problems it's a pretty good one. At the other end of the spectrum is the challenge of providing reliable service to small cities or sprawling suburbs where residential density and ridership demand are very low.

Most buses operate on a fixed-route system: they pass through a series of predetermined stops that passengers must get to and from on their own. In densely populated places, where the closest bus stop is never too far away, the fixed-route system can work well. But in low-demand areas fixed routes can be terribly inefficient. Stops are spaced so far apart that it can seem like you need a bus just to get to the bus stop. On nights and weekends, or during bad weather, this inconvenience is magnified.

Yanfeng Ouyang, a civil engineering professor at the University of Illinois at Urbana-Champaign, recently set out to address the problem of designing a bus system suitable for low-demand areas. What emerged was an entirely new transit concept that combines the regularity of a fixed-route bus system with the range of a car service. He calls it a "flexible-route transit system."

"The best way to think about it is a combination of the two: the fixed-route and the dial-a-ride," Ouyang says. "So we combine these two and take advantage of both of them in this idea."

Ouyang describes his system in detail in a paper written with Seyed Mohammad Nourbakhsh in the January 2012 issue of Transportation Research Part B. As conceived by the authors, the flexible-route bus would move within a coverage area called a "bus tube." The bus moves back and forth through the tube, passing transfer points along the way. That enables it to maintain the regularity of a traditional bus service.

Unlike a fixed bus, however, the flexible-route bus would pick up and drop off passengers at their precise origins and destinations within the tube's service area. People who need to reach a destination outside the tube are dropped off at transfer points and picked up by another bus that completes the route. The resulting movement would look something like this:

The system would require an information infrastructure similar to that of a dial-a-ride service. Potential passengers would go to a website — or call a central number — and input their point of origin and destination. A dispatcher would alert the nearest driver of the request.

Though the flexible bus may sound a bit confusing on paper, from the passenger's perspective the system is no different from a car service. The total ride might take a bit longer than a cab would, but passengers would be paying less. The service is also much more convenient than a typical bus; passengers no longer have to walk great distances to the nearest stop or, once there, wait great lengths of time for the next bus to arrive. From a passenger desirability standpoint, the system rates quite high. 

In their recent paper, Ouyang and Nourbakhsh compared the cost of their flexible-route system to a fixed-route bus and a taxi service. They found that a standard car service would be desirable only at extremely low levels of demand, somewhere below 4 passengers an hour. Fixed-route buses, as we established, are more cost-effective at great densities. The flexible bus was most cost-effective in the low-to-moderate demand range of about 4 to 40 passengers per hour per square kilometer.

"Think about Chicago downtown: There's very high demand, so there's no incentive to do our system," says Ouyang. "The passengers would not need to walk long distance anyhow. The traditional system would be OK. When demand is lower than a certain threshold our system would be much better. Otherwise, if we do a fixed route system, the distance between the bus stops will be very far, a mile or half a mile, and in a snow storm, or at night, people don't want to do that. That's the whole purpose."

Ouyang believes the flexible-route bus can establish itself on a college campus like Illinois and expand its appeal from there. Helping his cause is the fact that his system has a very low cost of implementation, since it requires no additional lanes or bus stops; the greatest requirements are a central dispatcher, and perhaps some driver training. Perhaps the biggest challenges to implementing the idea is getting people to accept the change, but if worked into a low-demand community gradually — beginning with nights and weekends, for instance — it could be a logical fit.

"In my opinion, it's going to come very naturally in the future," Ouyang says.

Photo by Robert Galbraith/Reuters

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