Several studies are available on so-called optimal agricultural field length. Boss and Flury  cite a Swiss study from the early 1980s. This study shows that fields suitable for arable farming require a road on both sides, whereas one road is sufficient for grassland.
This difference is explained by the burden of the harvest. Instead of a second public road, private farm roads on the field may be an alternative in arable farming. Further, Boss and Flury  state that the field lengths are dependent on the cultivated area per farm, the degree of mechanization, the soil utilization system, and the crop rotation. Figure 3.7 demonstrates the various effects of these factors on a 16-ha farm. The optimum field length is based upon the cost of turning (related to reduced yields at field edges and in the turning area), "empty" trips (due to additional filling and emptying during fertilizer application, spraying, and harvesting), double work during turning at field edges, and the additional expenditure of seed and fertilizer during an entire crop rotation. In addition to the 16-ha farms in the figure, it can be summarized that for 9-ha farms and 25-ha farms, respectively:
• the optimum field lengths are about 250 m and 450 m, respectively;
• the associated road densities are 3.5 and 2.5 km/km2, respectively;
• the optimum ratio of field width to field length is 1:5 or 1:4, respectively.
It is clear that the optimum field length increases and the optimum road density decreases with increasing farm size.
To calculate the optimum road density in agricultural areas, it is advisable to take more factors into account than the agricultural ones mentioned above. Surely, factors such as costs of construction and maintenance of roads and ditches, as well as costs of drainage systems, should be considered. Then, an optimum size for arable farms appears to be 300 x 1,000 m for a 30-ha farm, 500 x 1,200 m for a 60-ha farm and 500 x 1,700 m for a 85-ha farm .
Generally, it can be stated that the larger the farms, the larger the fields and the lower the road density. This holds for public roads. Depending on the bearing power of the soil, private paved farm roads are advisable for larger farms in order to avoid long distances for internal transports on bad subsoil.
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