Very easy to say, but would you want to be the Air Traffic Controller that has to accept the responsibility for thousands of lives at a time? The job itself, despite the help from technology, is pretty stressful as it is without adding to the burden.

The Ground controllers would have a nightmare too, with taxiways being used for mixed mode. It would be extremely hard for them to keep up to speed with who's lining up for take-off, who's waiting to go to a stand and who is under tow. The single mode operation means a neat, orderly, methodical way of working - mixed mode calls for much greater powers of concentration and would make handing over at the end of a shift almost an impossible task.

Mixed mode runways work well at less crowded airports. Heathrow, thanks to successive short-sighted politicians and the "NIMBY" brigade, is probably the most congested airport in the world.