Our grip on the steering wheel lessens, our muscles relax, and overall we feel more comfortable. Our eyes are being allowed to work as nature intended. Our pupils dilate and adapt to the lower light levels.
Contrasts become much more distinct.
We're guided along our way not by glaring street lights, but by our headlights, the reflecting stickers attached to posts at the sides of the road, and by the reflectors that are often embedded in the pavement. Compare this to city driving. Our eyes are assaulted by bright and competing light sources. Our pupils constrict to adapt to the brightest of these lights, and our ability to see into the shadows is nonexistent. We are far more tense and more prone to becoming impatient and discourteous drivers.