40 BULLETIN" 724, U. S,' DEPARTMENT OF AGRICULTURE. 
sure on the road surface of about 5 tons under each rear wheel. The 
use of much heavier equipment is in sight, however, and unless some 
regulations be passed to prohibit it the occurrence of 10-ton or even 
heavier motor trucks on our local highways adjacent to large cities 
or between large centers that are only a few miles apart may become 
common. 
Slight irregularities in the surface, due to faults in construction 
or developed by the wearing action of traffic, have the effect of 
producing impact under moving wheels and this action serves sub- 
stantially to increase the pressure to be distributed by the road crust 
and foundation. On account of the impact which they produce 
swiftly moving motor trucks are sometimes more severe on road 
foundations than the heaviest traction engines, and if an improved 
road is to be subjected to any considerable volume of motor-truck 
traffic the impact feature should be given especial consideration when 
the road is designed. 
In asuming the maximum wheel load for any particular road a 
reasonable allowance should be made for future increase. Since 
motor trucks have come into use there has been a constant tendency 
to increase both their rates of speed and the loads the}^ carry, and 
it is quite probable that this tendency will continue. Furthermore, 
many roads not now subjected to motor- truck traffic will attract such 
traffic after they are improved, and this possibility always should be 
considered. Increase in the volume of traffic also may be an im- 
portant factor, because a foundation ample to support an occasional 
heavy load might fail soon if the same load was often repeated. 
DISTRIBUTION OF PRESSURE. 
The manner in which pressure due to concentrated loads is dis- 
tributed by the various materials used in road construction never 
has been fully investigated, and any assumptions regarding this 
point that may be made at present would be purely conjectural and 
of doubtful value. In the past engineers have made the questions 
of when a foundation should be employed and of how it should be 
designed wholly matters of judgment, and few, if any, rules have 
been formulated to aid even in making the judgment consistent for 
different conditions and materials. This lack of method is due 
directly to the absence of accurate knowledge- concerning the cone of 
dispersion and pressure distribution under concentrated loads, and 
until further experimental data are obtained the design of road 
foundations necessarily must be based on rather uncertain empirical 
information so far as comparing the efficiency of different materials 
and methods of construction is concerned. 
