Traffic and the Lake Street Bridge...
Here are some of the results.
I took a couple dozen shots, trying to time the traffic movement to maximize the effect, but this turns out to be harder than I thought when working with a 2 second timer and a long shutter speed.
The next two are taken from approximately the same position. The second one has less traffic, but the Green Line is approaching overhead.
Lastly, the Metra tracks below the bridge which were carrying traffic below me.