c should be the
limiting consideration for main trunk line highways, but it is
certain that for a number of years horse drawn traffic will be a
factor on secondary roads.

In the case of motor vehicles, excess power is provided by means of
gears and no difficulty is encountered in moving vehicles over grades
up to 12 or 15 per cent, so that any grade that would ordinarily be
tolerated on a main highway will present no obstacle to motor
vehicles, but the economy of such design is yet to be investigated.

=Energy Loss on Account of Grades.=--Whether a vehicle is horse drawn
or motor driven, energy has been expended in moving it up a hill. A
part of this energy has been required to overcome the various
resistances other than grade, and that has been dissipated, but the
energy required to translate the vehicle against the resistance due to
grade has been transformed into potential energy and can be partially
or wholly recovered when the vehicle descends a grade, provided the
physical conditions permit its utilization. If the grade is so steep
as to cause the vehicle to accelerate rapidly, the brakes must be
applied and loss of energy results. The coasting grade is dependent
upon the character of the surface and the nature of the vehicle. In
the cases discussed in the preceding paragraph, the coasting grades
would be five per cent and three per cent respectively. For horse
drawn vehicles then the economical grades would be three and five per
cent, which again emphasizes the necessity of lower grades on roads
that are surfaced than on roads with no wearing surface other than the
natural soil.

The theory of grades is somewhat different when motor vehicles are
considered, since it is allowable to permit considerably higher speed
than with horse drawn vehicles before applying the brakes and the
effect of grade can be utilized not only in translating the vehicle
down the grade, but also in overcoming resistances due to mechanical
friction and the air. On long grades, a speed might be attained that
would require the use of the brake or the same condition might apply
on very steep short grades. There is at present insufficient data on
the tractive resistance and air resistance with motor vehicles to
permit the establishing of rules relative to grade, but experience
indicates a few general principles that may be accepted.

If a hill is of such rate of grade and of such length that it is not
necessary to use the brake it may be