Chapendra / Flickr

A brief lesson on how bus and rail connections save time.

A classic public transit lesson from Jarrett Walker is getting some renewed attention at CityMetric, and deservedly so. Walker uses two infographics to explain why people should reconsider their hatred of making bus or train transfers. In demanding direct, non-transfer service, he writes, "you may actually be demanding a mediocre network for your city."

Let's take a look at the first image—a direct, non-transfer system:

(Jarrett Walker)

Direct service in this system connects three residential areas to three commercial areas, for a total of nine transit lines. Assume this hypothetical city can afford to run transit service every 30 minutes on these routes. That means, on average, a rider will wait about 15 minutes for the bus or the train. If you take that wait time, and add it to a travel time of (say) 20 minutes, then the average trip takes 35 minutes.

Now let's take a look at the second image—a system that relies on transfers:

Jarrett Walker

This transfer-heavy system only requires three lines to connect the city's commercial and residential areas. That means it can run service three times as frequently: every 10 minutes, with an average wait time of 5 minutes. So now the average trip becomes 5 minutes of wait time, 10 minutes of travel time (halfway to the destination), another 5 minutes of wait time at the transfer point, and another 10 minutes of travel time (to reach the destination).

Altogether that's a total average trip time of 30 minutes—5 minutes better than the direct-service system. And the more a city grows, the more time advantages a transfer system delivers. In a city with twice as many residential and commercial areas, for instance, the direct service still takes 35 minutes but the average transfer service is down to 25 minutes.

This thought experiment does have its limitations. The rise of real-time transit information should reduce wait times for passengers and thus reduce direct-service averages as a result. The design of transfer points also matters; the transfer system above doesn't include additional walking time to make a connection, which might be several minutes if riders need to do anything more than cross a platform.

But there are other benefits to a transfer-heavy system besides time, including better coverage; here's what emphasizing transfers (among other changes) did for the scope of Houston's bus system:

Houston Metro Transit

If that's what comes of liking transfers, it's hard to see anyone hating them.

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