Just a simple shot, really - a few seconds when the traffic is whizzing by!
I have grown to love long exposures and especially light trail photos. They are easy to do and often come out great, or at least interesting. I just set up in a spot where I know there will be passing traffic, and wait. Once I see traffic coming, I start a long exposure and hope for the best! Sometimes it takes a few tries, but usually I get something useful.
This photo was a combination of a double-decker bus, some cars and a taxi or two. That's what is so great about doing these in London - the busses! Their lights are higher than on cars, obviously, so it helps fill the frame with more light trails. Fun, right?!
It also helps that St Paul's Cathedral is in the background. If I can find a great background, it's just a matter of time before the traffic streaks by, and I can capture it.