Cold weather and darkness are not obstacles. Here in Finland the winters are darker, colder, and longer than in the vast majority of the US, yet even elementary schoolers usually go to school by themselves, often by foot or on a bike, sometimes by taking a bus or a train.
The problem that the US faces with respect to this issue is primarily caused the design of american cities (including the surrounding suburbs), which are laid out in a manner that makes the use of a car a practical necessity for getting anywhere. You wouldn't have to worry about a 20-mile country road to school out of a suburb if you instead made the sensible choice of placing services (like schools) right where they are needed, as european cities tend to do, instead of 20 miles away.
The problem that the US faces with respect to this issue is primarily caused the design of american cities (including the surrounding suburbs), which are laid out in a manner that makes the use of a car a practical necessity for getting anywhere. You wouldn't have to worry about a 20-mile country road to school out of a suburb if you instead made the sensible choice of placing services (like schools) right where they are needed, as european cities tend to do, instead of 20 miles away.