3 feet is stupid. At low speeds it's more than is required, and high speeds it's too little.
The "motorist safety" arguments against it are absurd, however, as John just pointed out: "Completely missing from the article is the simple observation that no one is forcing anyone to make an unsafe pass." If you can't slow down in time for a cyclist up ahead, then you're driving too fast, by definition. If you want to drive that fast, then get on a freeway, where cyclists and other slow moving vehicles are banned (except for shoulders in some sections).