Taxis and congestion charge - Dynamic Dave
Because this is Ken's main source of transport. Wasn't there a report in the papers that he spends £5k a year on taxi fares?
Taxis and congestion charge - Baskerville
Presumably because taxis are in constant use. For example, ten fares into the zone means, say, ten people and one car. With private cars ten trips means ten people but also ten cars, possibly all trying to be in the same place at the same time, ergo, more congestion. Cuts down on the parking spaces required too. Simple, really.
Taxis and congestion charge - Ben79
But it is not supposed to be a parking charge, hence being free if you don't move the car.

Ben
Taxis and congestion charge - Dave_TD
The taxis are going to be there anyway, there are already plenty of people that use them. If the charge encourages more people to use a cab (or bus, or tube) rather than take their own car into the city then it means less cars on the road.
On an average day I'll do 40 jobs or so, two thirds of them being around 1 or 2 miles. That means my nice warm diesel car doing 60mpg and emitting 149g CO2/km is polluting the atmosphere a lot less than the customer's stone cold Sierra, BMW 7-series, Range Rover or what have you. Also I help the traffic flow, because I don't stop on the Keep Clear boxes on roundabouts, or queue to get into car parks, or do three circuits of the same street looking for a space.
Of course, the shame is that because I'm not based in London, I don't get exemption from the charge.
Taxis and congestion charge - Ben79
Good point about your car engine being warm, but in terms of congestion, a taxi uses the same space as a private car.
Taxis and congestion charge - Baskerville
But Ben, cabs are in constant use, so you get many trips out of one cab in a day, but only one from a car used for commuting--not an efficient use of limited resources, of which road space is one. Every cab journey is equivalent to one private car on the roads so it takes the amount of space covered by, say, thirty private cars to carry the number of people who use one cab in the course of a day. So in this case private cars would take up thirty times the amount of road space[1] (parking is an important but secondary spin-off benefit) and create a lot more congestion. There was something on the radio this morning about how Londoners have already cut down on private car use in the run up to the charge.

[1] Figures hypothetical and for example only.
Taxis and congestion charge - DavidHM
I'm with Ben on this one. They would take up 30 times the road space, if the drivers were stupid enough to park in the middle of the road. Assuming they don't, and have somewhere to park, they're out of the way for the rest of the day anyway. Anyone who drove into central London not knowing where they were going to park would be naive at best.

In any case, cabs (not very far for black cabs in London, admittedly) go to the next job/back to base empty. A cab into work, and one home, is 4 journeys; driving is only two.

Of course, the benefits of cabs in making people think twice about unnecessary journeys, keeping sober behind the wheel, driving reponsibly (overall anyway), knowing their way around back streets and spreading out the traffic density, and using less resources over their life than more private cars doing the same mileage, mean that we should encourage them - plus it's only logical if they can use bus lanes that they should be exempt from the charge too.
Taxis and congestion charge - BrianW
"If the charge encourages more people to use a cab (or bus, or tube) rather than take their own car into the city then it means less cars on the road."

I may mean less cars, but the same number of vehicles, surely.

If Joe Bloggs takes his car from A to B that is one vehicle on the road for the duration of that journey.

If Joe Bloggs takes a taxi from A to B that is one vehicle on the road for the duration of that journey.

The only difference is that Joe Bloggs does not have to find a parking space.

Traffic is only reduced if Joe Bloggs shares his car or taxi with someone else or switches to bus or train.
Taxis and congestion charge - CM
Not that I support Ken, but he did have a point that it was cheaper than having an official car. (Champagne socialist me think!)