Motorists are used to cyclists being slow because of this overly cautious approach that novice cyclists take to riding bikes on roadways. Another way to help retrain motorists' expectations is to do better to keep up with traffic. If you are causing traffic blockages while on a bike, then you are not riding fast enough. There are multiple ways to address this, the simplest (though not easiest) being improving your cardio and strength. A more long-term, better solution would be to redesign infrastructure either to keep cyclists separate from motor traffic or to make motor traffic slow enough that bikes don't seriously impede traffic flow when they take the lane.
Whether a cyclist can maintain 20 mph or just 10 mph isn't going to make a difference to the motorists. But taking the lane by default will train them to change lanes to pass like they would when encountering any other slow vehicle.
That's what I'd propose. Carving up the already scarce roadways to try and squeeze in a "safe" lane for cyclists is going to make it far more dangerous for both bikes and cars. Bike lanes that run above the street would be far safer.
