Traffic in central London moves at the same speed as horse-drawn carriages
The 1800s have called...The theoretical maximum speed of all kinds of things is constantly going up; cars, trains, planes, boats... But in practice, we don't always get anywhere near those speeds. A good example of this is how, despite a century of technological progress, vehicles in London's city center are moving at average speeds comparable to the horse drawn carriages of yore (which also caused traffic jams), or about the same as a running chicken...
And that 10 MPH average is after a 1.5 MPH bump caused by the implementation of the congestion charge in 2003 (which is a 15% improvement -- nothing to sneeze at).
While London's congestion charge was certainly a good idea and things would no doubt be worse without it, something that the critics don't seem to take into account, the city's experience shows that it wasn't a panacea. Cities everywhere need to take more comprehensive approaches to making their cores less congested and more livable.
Image: Press-Office City of Müenster, Germany
Incentives to keep vehicles that shouldn't really be there in the first place out are a good idea, but they need to be supplemented with good urban planning, fast and convenient transit options, safe and accessible bike infrastructure, and lots of pedestrian space. It's the basics that we've been talking about forever...