what prevents the traveler from seeing farther down the road ?
There are several things that could inhibit one from seeing farther than the limits of ones vision;
1) Vision problems: People that are near sighted can only see what's close to them. This may be the result of a degenerative optical condition or disease. It might also be the result of a birth defect. With corrective lenses this problem can be addressed.
2) Pollution: Dirt, ashe, exhaust or other pollutants can obstruct vision. This is most apparent after a rainy day, when air quality improves. In L.A. the San Gabriel mountains are only visible after the rain cleans pollutants out of the air, greatly improving the view.
3) Topography: Hills, mountains or the natural contours of the landscape can obstruct ones field of vision. Vegetation can also do this.
4) Air Density: Even free of pollution, you can only see so far due to the naturally occuring particulars in the air. Dirt, pollen and more dense gasses are all part of the phenomenon.