dangerous turns a solid plank wall six or eight feet
high is sometimes built of such substantial construction as to
withstand the severest shock without being displaced.

Trees, shrubs and the berms at the side of the road in cuts are
particularly likely to obstruct the view and should be cleared or cut
back so far as is necessary to provide the proper sight distance.

=Width of Roadway.=--For roads carrying mixed traffic, 9 feet of width
is needed for a single line of vehicles and 18 feet for 2 lines of
vehicles. In accordance with the above, secondary roads, carrying
perhaps 25 to 50 vehicles per day, may have an available traveled way
18 feet wide. Those more heavily traveled may require room for three
vehicles to pass at any place and therefore have an available traveled
way 30 feet wide. Greater width is seldom required on rural highways,
and 20 feet is the prevailing width for main highways.

=Cross Section.=--The cross section of the road is designed to give
the required width of traveled way, and, in addition, provide the
drainage channels that may be needed. In regions of small rainfall the
side ditches will be of small capacity or may be entirely omitted, but
usually some ditch is provided. The transition from the traveled way
to ditch should be a gradual slope so as to avoid the danger incident
to abrupt change in the shape of the cross section. The depth of ditch
may be varied without changing to width or slope of the traveled part
of the road as shown in Fig. 10.

[Illustration: Fig. 10]

=Control of Erosion.=--The construction of a highway may be utilized
to control general erosion to some extent, particularly when public
highways exist every mile or two and are laid out on a gridiron
system, as is the case in many of the prairie states. The streams
cross the highways at frequent intervals and the culverts can be
placed so as effectually to prevent an increase in depth of the
stream. This will to some extent limit the erosion above the culvert
and if such culverts are built every mile or two along the stream,
considerable effect is produced.

Where small streams have their origin a short distance from a culvert
under which they pass, it is sometimes advisable to provide tile for
carrying the water under the road, instead of the culvert, and, by
continuing the tile into the drainage area of the culvert, eliminate
the flow of surface water and reclaim considerable areas of land.

Erosion in the ditc