My heart goes out to everyone affected by the floods. It can be devastating on so many levels, individual lives lost; extensive property damage to individual residences, and infrastructure damage. I managed the system in Ames, IA during our floods in 1990, 1993, 1998, and 2005 and I have been assisting the Cedar Rapids, IA transit system in recovering from their 2008 floods. I worked in Chicago during the blizzard winters of 1977-78 and 1978-79 where the city and suburbs experienced several weeks of paralysis due to the continuous heavy snows as well as way too many blizzards in Iowa over the years.In the U.S. buses typically operate without a lot of redunancy in the route network. Your commentary on numbering overlapping bus routes and make them understandable to the riding public is interesting and implies that there are lots of routes serving several corridors. In the U.S. that may be true, but usually the headways are pretty well trimmed to provide the absolute minimal level of service. When you add demand due to adjacent services becoming inoperative, the existing routes are overwhelmed. Overloading causes extended travel times and buses cannot make their normal cycle times which exacerbates the problem. Throw in a street network in chaos and the bus system will be criticized as not meeting the needs of the citizens in the time of crisis. No easy way to explain the problem.The other problem that we had in Ames, Chicago, and Cedar Rapids was that some of the drivers lived in areas that were flooded or had immediate family in those areas. They needed to tend to their family/housing priorities and this decreased the number of people available when the workload of more passengers and longer travel times increases.At some time in the future, after everything settles, perhaps you could solicit comments on building redundancy into your transit system. Sometimes, it is good to have lightly used routes that can be cancelled in a crisis allowing redeployment of drivers and vehicles. Sometimes it is good to have headways with a loading standard of less than 125% of seats at the peak point on the route instead of cramming buses with 150% or 175% of seated load. Your 10 or 12 minute frequent headway concept can provide additional resources if you need to cut it back to 15 to 20 minutes during a crisis.After the September 11, 2001 disaster in New York, the subway system was able to recover quickly because the lower end of Manhattan was one of the few places in the subway system where there was some redundancy. Multiple routes close to each other and at the end of some routes made it easy for commuters to resume their normal lives long before the reconstruction of the subway damage.Redundancy is not favored by policy makers and can add to costs. However, a system with excess capacity will perform well in times of crisis and will provide addtional service during normal times.Our prayers are with everyone who is suffering through this disaster and we hope that good luck will shine on Australia again.
The kind of redundancy that Bob praises is something transit planners spend much of their time trying to get rid of, because on typical days when we don't need it as redundancy, we call it duplication and waste.
Transit agencies work on such tight budgets that it doesn't make sense to run, say, a bus line next to a rail line, doing the same thing, just for the redundancy. If the rail line is serving the market, the bus should be off somewhere else, providing unique mobility rather than duplicating the rail.
This is especially true in small cities like Bob's hometown of Ames, Iowa, or Great Falls, Montana. These networks' resources are stretched tightly to create the maximum amount of mobility for the budget.
Having said that, there are a few situations where an efficient network is also a redundant one.
Classic high-frequency grids provide redundancy for transit in the same way they do for cars. If one segment in a grid goes down, there's a parallel line 800m away that you can walk to in a pinch. It will probably allow you to complete the same L-shaped trip that you intended to make on the disabled line.
Ferries are more complicated. The long cross-city run of the CityCats mostly connects stations that are also connected by bus. The bus trip generally runs a shorter distance at a higher frequency, though it may require a connection. So there's no question that the intrinisic attraction of the ferry is part of what keeps it busy. There may also be secondary issues, like the legibility problems of much of the bus system in downtown Brisbane, where most connections occur. But there are also situations where CityCat and the smaller CityFerry does a link that's simply impossible by road, or much, much longer, and in these cases the ferry wins on pure mobility grounds.
Of course, bus operators generally have backup fleets in case they need to suddenly replace a non-redundant train or ferry line that goes down. In Australia, there are often standing agreements between government and private operators to shift buses into this role, and given a day's warning — which Brisbane had — it's not hard to replace a failed network segment with buses even while running the rest of the bus network.
So is redundacy a good thing in disasters? Of course it is. Is it a reason to design networks that are redundant all the time, at the expense of more mobility that could be provided at the same cost? No, probably not, because you're weighing a rare disaster against daily inefficiency. Are there styles of network design that are both efficient and redundant? Yes, the high-frequency grid comes to mind.
In really big and dense cities, you can also get both redundancy and efficiency, because there will tend to be overlaps of service just to provide capacity into dense centers like Lower Manhattan, and in these cases, as Bob notes, redundancy is often possible. The key there, however, is that the duplication of services isn't justified by the need for redundancy, but rather for the sheer capacity need, and providing necessary capacity, of course, is part of efficiency.