On yesterday's post on the watering-down of Bus Rapid Transit proposals in Bristol, UK, a number of comments seem a little vague about how buses are more "flexible" than rail. For example, Carl writes:

The two primary justifications for BRT [are]: 1. Like rail but cheaper. …

2. More flexible. More flexible means it doesn't need exclusive right of way everywhere (usually the chokepoints), that other traffic can use the lanes, etc.

This a very common way of framing the question, and a very misleading one.

First of all, the notion of "flexibility" used in #2 has nothing to do with the bus/rail distinction, at least if we're talking about surface light rail or streetcars. You can put streetcars or light rail in mixed traffic and get all the same speed and reliability problems that a bus would deliver in the same situation. So again, in urban transit: Speed and reliability are not about vehicle technology; they are about what can get in your way.

But Bus Rapid Transit offers a very different flexibility that in certain situations out-competes rail. A busway can be designed so that buses from many surface lines can flow into it. This potentially spreads the usefulness of the busway over a large area without requiring an additional trunk-to-feeder connection. Connections are unavoidable in good networks, but if there are easy opportunities to eliminate one, it's still worth going for.

This ability to flow through to local lines yields what we call an open busway. North American open busways include the Ottawa busway network, the Pittsburgh busway, and the Los Angeles El Monte Transitway, but they are much more common overseas, including the developed world's most extensive example in Brisbane, Australia. This kind of flexibility is impossible to do with rail.

Closed busways, which has none of these benefits but can have more "specailized" vehicles, include the Los Angeles Orange Line.

The flexibility of open busways makes sense only where it matches the pattern of the market. Brisbane is a highly radial city, with a single downtown and densities dropping away as you move away from it. Outlying nodes of high activity, which could be a strong endpoint for a closed busway or rail line, are scarce. So the open busway makes perfect sense. It allows busway service to spread out over a larger area, yielding high frequencies on the inner busway where the demand is higher, and correspondingly lower frequency further out. It's the kind of flexibility that fits the city.