"Riding double file deters unsafe same-lane passing in narrow lanes and makes the cyclists as visible as a car from the front and behind."
"When cyclists ride near the right edge of a narrow lane, an overtaking motor vehicle driver may misjudge the remaining space in the cyclists’ lane, and fail to change lanes. This is the most common cause of car-overtaking-bicycle collisions in daylight. But when cyclists make it clear from a distance that a same-lane pass is not possible, motorists slow down earlier and plan their lane change sooner, reducing the cyclists’ risk of a sideswipe."
Thanks to BikeWalkNC for excellent article @ http://www.bikewalknc.org/2015/04/why-cyclists-ride-two-abreast/