class of traffic.

=Tractive Resistance.=--The adoption of a policy regarding the grades
on a road involves an understanding of the effect of variation in the
character of the surface and in rate of grade upon the energy required
to transport a load over the highway. The forces that oppose the
movement of a horse drawn vehicle are fairly well understood and their
magnitude has been measured by several observers, but comparatively
little is known about the forces opposing translation of rubber tired
self-propelled vehicles.

The resistance to translation of a vehicle is made up of three
elements: resistance of the road surface to the rolling wheel,
resistance of the air to the movement of the vehicle and internal
friction in the vehicle itself.

=Rolling Resistance.=--When the wheel of a vehicle rolls over a road
surface, both the wheel and the surface are distorted. If the wheel
has steel tires and the road surface is plastic, there will be
considerable distortion of the road surface and very little of the
wheel. A soft rubber tire will be distorted considerably by a brick
road surface. Between these extremes there are innumerable
combinations of tire and road surface encountered, but there is always
a certain amount of distortion of either road surface or wheel, or of
both, which has the same effect upon the force necessary for
translation as a slight upward grade. When both the tire and the road
surface strongly resist distortion (as steel tires on vitrified brick
paving), the resistance to translation is low but the factor of impact
is likely to be introduced. Where impact is present, energy is used up
in the pounding and grinding of the wheels on the surface, and this
factor increases as the speed of translation, and may be a
considerable item. Impact is especially significant on rough roads
with motor vehicles, particularly trucks, traveling at high speed.
These two factors (impact and rolling resistance) combined constitute
the major part of the resistance to translation for horse drawn
vehicles.

=Internal Resistance.=--For horse drawn vehicles, the internal
resistance consists of axle friction, which is small in amount. For
self-propelled vehicles, the internal resistance consists of axle
friction and friction in the driving mechanism, of which gear
friction and the churning of oil in the gear boxes is a large item.
Internal friction is of significance in all self-propelled vehicles
and especially so at h