ruts are pretty rough. go light in the handling. the bike knows where it wants to travel in the ruts.
the following doesn't apply to the ruts but applies to any slick pavement and white, blue or black ice.
Learn skid turns. under steering is dangerous in slick conditions. if you're not running studs, better to throw the rear end out agressively, get the inside foot sliding along, and skid turn when its uberslick. you'll soon be able to balance on your bike, foot out but not touching the pavement, as you slowly countersteer a graceful, skidded arc into your new desired direction of travel.
I grew up riding in the Upper Peninsula of Michigan, and skid turns on ice, greazy pavement, and packed snow is the way to go.