Mulitiple elevators per shaft

One problem with tall buildings is having enough elevator capacity. The obvious solution is to have lots of elevators, but as buildings become taller, a larger precentage of the square footage must gets used for elevator shafts in order to maintain the same level of service.

If each shaft was dedicated to either up or down (this was inspired by the details of [AO's] Private Elevator idea, but is applied here for slightly more conventional elevators), then there could be more than one elevator per shaft. So the taller a building is, the more vertical space there is in each shaft, allowing more elevators to share the same shaft. This lets the elevator system scale up with the building height much better.

The elevator mechanism would be quite different from a conventional elevator, so designing the system along with all the appropriate safety systems could be expensive, but it wouldn't need to use any exotic technology.

The control of the elevators would be an interesting problem as well. The elevators should be spaced far enough apart and the stops coordinated so one elevator doesn't have to stop and wait for another unless someone blocks the door for too long. In that case the following elevator would need to gently slow and/or stop, preferably informing the passengers of the reason for stopping. If one elevator got stuck or damaged (or the door was being held for a very long time), the other elevators sharing the same shaft would need to inform the passengers and let them out so they could find a shaft that wasn't blocked. This probably wouldn't be practical for a building with only two safts, but if it were used, and one shaft got blocked, all but one elevator would need to hang out in the blocked shaft while the remaining elevator went up and down the one unblocked shaft like a standard elevator. The elevator control system would probably apply many existing techniques such as an expess shaft that will skip some range of floors, and HB "Intelligent Elevator" technology to avoid stopping to pick up more passengers when the elevator is full.