12 BULLETIN 1077, IT. S. DEPARTMENT OF AGRICULTURE. 
necessary width for a double-track pavement and a 9-foot pavement 
built to one side of the center line of the grade. In widening a pave- 
ment of this type to 18 feet it would only be necessary to lay a slab 9 
feet in width adjacent to the original slab. A typical cross-section 
for a pavement of this type is shown in Figure 7, page 28. 
The character of vehicles, together with the clearance necessary 
for safety in passing, will largely determine the width of pavements 
for double-track roads. Motor-truck traffic has grown to such pro- 
portions that it has been necessary in many States to limit by statute 
the size of load and the total width of body. The maximum width 
of truck body generally permitted is 8 feet. If ample clearance is 
provided for the passing of trucks of maximum size a desirable 
factor of safety will be provided for smaller trucks and passenger 
motor vehicles. For slow-speed traffic, such as truck traffic, a clear- 
ance of 3 to 3J feet is necessary for safety, while for high speed 
traffic, such as automobiles, a clearance of at least 5 feet should be 
provided. The amount of truck traffic is small, in comparison to au- 
tomobile traffic, except in the neighborhood of large cities, so that 
the frequency with which one truck passes another is almost neglible 
in comparison with the frequency with which automobiles pass 
trucks. If, therefore, ample clearance is allowed for the passage of 
an automobile and a truck, the maximum of safety will be obtained 
at the minimum of cost. 
The diagram, Figure 1, shows the width of pavement necessary 
for reasonable clearance for trucks passing each other and for an 
automobile passing a truck. At an average speed of 30 miles per 
hour it is unreasonable to expect the driver of an automobile to drive 
with the wheels closer than 1J feet to the edge of the pavement. For 
trucks at an average speed of 15 miles per hour, this distance should 
not be less than If feet on account of the great width of the rear 
wheel. Inasmuch as a certain amount of truck traffic is to be ex- 
pected on all main country roads, the minimum width of pavement 
for this class of road should be 18 feet. Where the frequency with 
which trucks pass each other becomes a big factor, as in the neigh- 
borhood of large cities, the minimum width of pavement should be 
20 feet. 
THICKNESS OF PAVEMENT. 
The determination of the proper thickness of a concrete pavement 
for different kinds of traffic is a very complex problem in applied 
mechanics, and depends to a large extent on certain factors which at 
present are more or less indeterminate. In the first place, the loads 
acting on a pavement are not merely static loads, but are applied with 
considerable impact. This impact varies with the roughness of the 
