The actual cross drain spacing on four forest roads was compared with an existing guideline. In total, 184 drainage segments, defined as segments between cross drains, were identified. Of these' segments, 90 % had rubber water bars or dips, which were assumed to have installed during maintenance, at one or both ends, indicating that the cross drain spacing was adjusted during maintenance. The frequency distribution of drainage segments' lengths fitted a lognormal distribution. Although the percentage of drainage segments longer than 100 m was only 14 %, they occupied 37 % of the total road length, whereas those shorter than 50 m occupied only 25 %. It was suggested that the cross drains were not evenly added across the roads; however, they were selectively added where required. The average difference from the guideline was +19 m (standard deviation, SD-57 m), and the mode was +25 m. This gap can be because of differences in targeted erosion prevention levels; although the guideline aims to prevent minor erosion, the actual cross drain spacing aims to prevent severe erosion that can interfere with traffic. The aforementioned SD indicates that the appropriate cross drain spacing can differ by dozens of meters even among segments with the same slope.

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