"Green wave" is the slang term for setting traffic signal timing to allow traffic to proceed at an optimal speed.  For example, on Martin Luther King Jr. Blvd. in SE Portland, the signals are timed for traffic to move at 27 mph.  If you drive that speed, you'll hit the green wave.

There are other places where Portland's traffic engineers are setting signal timing to optimize traffic flow.  Recently, the signals on SE Hawthorne between the Hawthorne Bridge and SE 12th Ave were set to enhance traffic flow.  People traveling on bikes should notice that it's easier to travel to Ladd Ave without stopping then before the changes. 

Check out this video from the blog Copenhagenize about the Copenhagen green wave.  SE Hawthorne might just look like this way once it hits 80 degrees.