Great shot, and very well achieved!

Try make up some more of these.
I got some questions (or suggestions... call 'em what you want :p);
Have you considered trying doing this in HDR? I know its kinda hard with all the moving vehicles, but you could do the HDR from, lets say, 3 images or so, and then afterwards make up the vehicles in a 20 sec. shot. Combined those the right way and it could look awesome!

Just an idea.
Second; tried it on a highway or another road, a little more outside the city? If you could achieve an almost pitch black sky, and only having a few cars passing by it could make up some nice effects aswell.
Go ahead and lets see some more shots. Keep it up!