12 BULLETIN 1077, U. 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 3^ feet is necessary for safety, while for high speed 

 traffic, such as automobiles, a clearance of at least 5 feet should be 

 j)rovided. Tlie 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 

 automol)ile 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 1^ 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 



