Why does the Uniform Vehicle Code take away a cyclist's right to use most of the roadway, restricting him or her to the far right?
Considering (1) the numerous exceptions to the 'keep far right' law, (2) how often a cyclist encounters these exceptions*, and (3) that every exception is safety-related, answers which suggest that keeping far to the right makes the cyclist safer must cite scientific research for support.
*Consider this example. The average car, bus, tractor trailer, and cyclist are 6, 8, 8.5, and 2.5 feet wide respectively. The minimum 'safe passing distance' is 3 feet (or more) wherever it's defined by law. Assume the cyclist rides 1 foot from the curb. (8.5 feet + 3 feet + 2.5 feet + 1 feet = 15 feet) Any road less than 15 feet wide is, by legal definition, substandard width. And since every travel lane in a cyclist's city or town is probably 12 feet wide or less, the cyclist may use the full lane virtually everywhere.
Note: Here are some relevant passages from the Uniform Vehicle Code (of the United States) for quick reference. I omitted some wording for brevity's sake. Visit the linked PDF if you wish to read the entire passages.
"Every person... riding a bicycle shall have all of the rights and all of the duties applicable to the driver of any other vehicle under chapters 10 and 11, except as to special regulations in this article..." - Uniform Vehicle Code, 11-1202, pp. 168
"Any person operating a bicycle... shall ride as close as practicable to the right-hand curb or edge of the roadway except under any of the following situations: 1. When overtaking and passing another bicycle or vehicle proceeding in the same direction. 2. When preparing for a left turn at an intersection or into a private road or driveway. 3. When reasonably necessary to avoid conditions' including but not limited to: fixed or moving objects; parked or moving vehicles; bicycles; pedestrians; animals; surface hazards; or substandard width lanes that make it unsafe to continue along the right-hand curb or edge. For purposes of this section, a "substandard width lane" is a lane that is too narrow for a bicycle and a motor vehicle to travel safely side by side within the lane. 4. When riding in the right-turn-only lane." - Uniform Vehicle Code, 11-1205, pp. 168