Countdown Electronic Displays
"Countdown" is an electronic display system used at some London bus stops and (I would guess most) bus stations to try to give passengers real-time information about how long they'll be waiting for the next bus.
They haven't actually finished installing the system yet; this leads some people to complain that it's never worth trusting the displays since they'll tell you everything except the truth. They can be useful, though, if you bear a few things in mind.
The way the system works is that battery-powered beacons are placed on lamp posts along the route, and as a bus passes a beacon it receives a message from it. It re-transmits this location identifier to a central data centre, along with information about the speed that the bus is going at. The central computers use this data (along with historical data for the same route) to work out how much longer the bus is likely to take to reach stops further along its route. (Information in this paragraph comes from the Symicron and APTA sources below, may not be accurate, but sounds plausible.)
So for a given route to be featured on the displays at all, the buses used on that route must have had the relevant equipment installed, and the route itself must have been equipped with beacons. This is not yet the case for all of London's buses — TFL's "Countdown for London" page has some statistics on current and projected coverage.
As well as that, the "due" buses sometimes disappear from the display before they've actually arrived, so you can get to the stop, see that the next 220 isn't due for another 16 minutes, wait a minute or so, and there along comes a 220. I generally only trust the displays when what they're telling me sounds plausible given my knowledge of the route frequencies (which are generally displayed at the stop anyway).
My experiences of Countdown on the 266 route by North Acton Station highlight some technical issues. There are often phantom buses, which are displayed, but never arrive. A tell tale sign of a phantom bus is that "Southbound" is displayed instead of the bus's eventual destination. I only trust the display if the minutes counter is going down. Another observation is that the sign never shows 1 minute, but jumps from "2 mins" to "due". --IvorW
A friend of mine who worked on the early implementation of the system comments that the buses' beacons were originally placed in the drivers' cabs, for ease of maintenance; the drivers objected to "tachographs by the back door", and started a campaign of sabotage and data corruption, mostly by placing foil crisp packets and drink cans around the beacons. To prevent this, the beacons are now mounted in a housing on the roof of the bus directly over the driver's cab. --Roger