assumed that no energy loss
results so far as motor vehicles are concerned. Where there is no turn
at the bottom of the hill and the physical condition of the road
permits speeds up to thirty-five or forty miles per hour grades of
five per cent are permissible if the length does not exceed five
hundred feet and grades of three per cent one thousand feet long are
allowable. It is a rather settled conviction among highway engineers
that on trunk line highways the maximum grade should be six per cent,
unless a very large amount of grading is necessary to reach that
grade.

=Undulating Roads.=--Many hills exist upon highways, the grade of
which is much below the maximum permissible. If there are grades
ranging from 0 to 4 per cent, with a few hills upon which it is
impracticable to reach a grade of less than six per cent, it is
questionable economy to reduce the grades that are already lower than
the allowable maximum. It is especially unjustifiable to incur expense
in reducing a grade from two per cent to one and one-half per cent on
a road upon which there are also grades in excess of that amount. The
undulating road is not uneconomical unless the grades are above the
allowable maximum or are exceptionally long or the alignment follows
short radius curves.

=Safety Considerations.=--On hills it is especially desirable to
provide for safety and curves on hills are always more dangerous than
on level sections of road. Therefore, it is desirable to provide as
flat grades as possible at the curves and to cut away the berm at the
side of the road so as to give a view ahead for about three hundred
feet. Whether a road be level or on a hill, safety should always be
considered and the most important safety precaution is to provide a
clear view ahead for a sufficient distance to enable motor vehicle
drivers to avoid accidents.

[Illustration: Fig. 9.--Types of Guard Rails]

=Guard Railing.=--When a section of road is on an embankment, guard
rails are provided at the top of the side slope to serve as warnings
of danger, and to prevent vehicles from actually going over the
embankment in case of skidding, or if for any reason the driver loses
control. These are usually strongly built, but would hardly restrain a
vehicle which struck at high speed. But they are adequate for the
protection of a driver who uses reasonable care. A typical guard rail
is shown in Fig. 9, but many other designs of similar nature are
employed. At very