These are line shapefiles of traced road edges. They are not completely literal; some averaging is done, especially in the case of dirt roads that may be much wider in some spots than other. However, the relative widths do well reflect the relative size and importance of the road. In the case of most roads, the edge lines are traced at the edge of the pavement, but in the case of major highways, the edge lines are traced at the outer edge of the main (graveled) shoulder.