I think it's generally acknowledged that, statistically, intersections are the most dangerous place for cyclists. For instance, http://www.vehicularcyclist.com/wachtel.html states that in Palo Alto CA (the subject of the study) "Accidents at intersections accounted for 237 of 371 total bicycle accidents (64 percent), and 233 of 314 bicycle-motor vehicle collisions (74 percent)." I think any research would turn up similar numbers for anywhere.

Regarding the lane usage, doubling up is a vehicular cycling issue, and goes to the situation. If doubling up causes more traffic to back up behind the cyclist than would have happened otherwise, then it should be frowned upon. OTOH, often the lane is too narrow to share with a car, in which case the cyclist is not only allowed but advised to 'take the lane' to avoid being dangerously squeezed. Having taken the lane, riding side by side with another bike shouldn't be an issue. Likewise, it's not an issue where there's more than one lane going in the direction of travel. Of course, tripling up and/or crossing the center line is never correct. Except when passing. The real problem with doubling up is, drivers often see 2-abreast as holding them up, when in fact it doesn't. It makes the line of bikes they must pass only half as long; and if they have to pull out to pass anyway, it's no additional inconvenience over passing a single bike.

That says nothing about accidents in rural areas.