Most states model their traffic laws after the UVC (Uniform Vehicle Code). For the keep as far right as practicable law when traveling at less than the normal speed of traffic, the UVC lists several exceptions to that requirement.
One is the substandard width lane which is defined as a lane that's too narrow for a cyclist and motor vehicle to safely pass side-by-side within the lane [1].
Given that most surface street lane widths range from 10 to 12 feet, they're not wide enough for a motor vehicle to pass a cyclist safely while remaining in the lane. That means that, for all practical purposes, the keep as far right as practicable requirement is an exception rather than a rule.
One is the substandard width lane which is defined as a lane that's too narrow for a cyclist and motor vehicle to safely pass side-by-side within the lane [1].
Given that most surface street lane widths range from 10 to 12 feet, they're not wide enough for a motor vehicle to pass a cyclist safely while remaining in the lane. That means that, for all practical purposes, the keep as far right as practicable requirement is an exception rather than a rule.
[1] http://iamtraffic.org/wp-content/uploads/2013/01/UVC2000.pdf (page 168 ยง 11-1205-Position on roadway subsection 3)