Riding abreast or in pacelines--the law

Robert Mionske has a comprehensive look at the legal aspects of riding two or more abreast and in pacelines:

Now let's put the law into action. Your club is riding in Pennsylvania, at the same speed as other traffic, and you're riding a double paceline. Nothing to worry about, you're riding within the law. If you cross the state line into New Jersey, still nothing to worry about, you're still riding within the law.

Now let's say your club is riding in Pennsylvania, but at a slower speed than other traffic. If you're on a road with more than one lane in your direction, you may continue to ride in a double paceline, but must ride in the right-hand lane. Otherwise, if you're on a road with only one lane in your direction, you must ride single file, as close to the right-hand curb as practicable. Once your club crosses the state line into New Jersey, if you are still riding at a slower speed than other traffic, you must ride single file, as close to the right side of the road as practicable, regardless of how many lanes there are.

Most of the issues Bob discusses in the article are the same or similar in all states, including Missouri and surrounding states.

The wording of various laws is slightly different in each state and sometimes this has important ramifications.

For instance, in Missouri, "Bicyclists may ride abreast when not impeding other vehicles". The law does not specify two abreast or any other number abreast as is the case in most other states.

Bicyclists are released from the duty to ride "as near the right side of the roadway as safe" in cases where the lane is too narrow to share safely with a motor vehicle.

See the complete Missouri bicycle law here.