United Kingdom The classic photo in front of Big Ben, London

Nearly everybody does this type of photography in London, especially with Big Ben in the background. The great thing about these long exposures is, that everyone is always different, because the light trails are always mostly random and depend on so many factors.

Use your tripod and set it to a very low level to get that view towards Big Ben and still have the buses cover your entire frame if you want that effect over your whole image. Wait until a double-decker is coming and expose your image for about 8-10 seconds at f11, depending on how dark it's already. The blue hour after sunset is a great timing for this kind of photography because then all lights of the passing cars and buses are turned on.
To get the best results you should take care about places where the traffic is fluently passing by because you usually don't want to have any standing cars in your photo.

Big Ben is pretty easy to reach by the London Underground. Just leave the tube at Westminster station.