I think the debate on speed is about limited-access freeways, where no bikes or pedestrians should be, and where other vehicles won't be doing things like making u-turns, parallel parking etc. As someone who uses basically all the modes of travel that a complete street allows, I'm all for lowering the speed limits (especially through road design, not just a speed limit sign). But a highway is completely different, the only thing you have to worry about are other cars.