The only two times I hitched south from Portland, I did it in the city and I got picked up w/in 10 min.
You can access this spot from the Portland Streetcar eastbound, it's the SW Harrison St. stop.
When you get off, walk down the road the way the streetcar goes (SE), and about two blocks away is a road that feeds to I-5. There's enough shoulder to wait in and show your destination sign, and it's big enough for rides pull over -
and the traffic lights before & after your position here will keep them from going to fast.
The streetcar stop you want
here.
The waiting/pickup spot
here.
I caught a ride one Sunday @ 5pm, and most recently again at 6pm on a Thursday.