I really know nothing about snowmobiles. If you were on country trials, why would you leave them for the road? And if you did, why would you go out into the lane of potentially oncoming traffic? I just can't visualize it because I never did it. Would you just pull out into a road and ride your snowmobile along the highway? How unusual is that? How fast can a snowmobile go? Was this a silly little girl thing or is it something that experienced snowmobilers would try?
I don't know if you are familiar with Northwest Iowa (I grew up just a few miles from this accident), but there are no "country trails" as such. This is all privately owned land, family farms, most with hundreds of acres of fenced land used for raising corn and soybeans and grazing cattle. In the winter these fields are laying fallow and used only for hunting (pheasants) and winter recreation, such as it is. Snowmobiling on your own private land is a MAJOR winter activity thereabouts.
The roads there are not freeways, throughways, boulevards or city streets. Except for the occasional "farm to market" paved road, they are high quality gravel roads with deep ditches, capeable of safely handling 50-60 mph very light traffic, usually spaced one mile apart, which give access to these large parcels of private property used for farming. There is no need to go on any of these roads to snowmobile as the best snow is found OFF the roads since the roads are plowed.
So the mystery here is why the girls were on this country road. That article does not give this information, but we can guess maybe they were going being farms. Their parents should have absolutely prohibited even crossing the road. Maybe they did.