A bend in a haulage road, which may be horizontal, vertical, or both. On main haulage roads, curves may be 100 ft (30.5 m) in radius or more. With a good rail track and smooth curves, haulage speeds up to 20 mph (32.2 km/h) can be maintained. A useful rule for determining the minimum radius of curvature for tramming is R = 12 to 15 x W; for locomotive haulage R = 20 to 25 x W, where W is the maximum wheel base of the rolling stock or locomotive.