22 BULLETIN 586, U. S. DEPARTMENT OF AGRICULTURE. 
The traffic over this road varies considerably from season to 
season and has not yet been studied in detail. It consists for the 
most part of tourist automobiles going to and from Mount Vernon, 
which accounts for the extreme variability. It is planned to make 
periodic counts of the traffic, and to publish the results in future 
progress reports. ! 
The purpose of the experiments was to gain information regarding 
the results to be obtained by using certain bituminous materials in 
the surface treatment of gravel roads. Three different materials 
were used, and the road was divided into three sections, according 
to the kind of bituminous material in the surface. Section 1 extends 
from station 0+00 to station 23+55; section 2 from station 23 +55 
to station 85+45, and section 3 from station 85+45 to station 
109+00. Before taking up the description of the experiments sepa- 
rately, the data common to all three will be stated briefly. 
The work was done by contract between November 3 and Decem- 
ber 5, 1916, during which time there were only 15 weather working 
days. ‘The contract prices are given in the latter part of this report. 
In all of the experiments the road first was swept clean with a 
horse-drawn rotary broom, followed by men with push brooms. 
The bituminous material was applied under pressure of about 15 
pounds per square inch by means of a motor truck distributor of a 
capacity of 1,200 gallons. The spray line of the distributor was 
8 feet long and covered one-half the width of the road at a trip. 
The top-dressing for all of the surface treatments was obtained — 
from the Walker gravel pit and consisted of pit-run gravel from 
which the coarser particles had been screened out. The screening 
was done by hand, and the screen used had approximately 1-inch 
opening and was inclined at an angle of about 45°. As shown by the 
mechanical analysis of this material, given in Table 25, comparatively 
few particles passed the screen that would not pass a 4-inch mesh 
laboratory screen. The gravel top-dressing was hauled out in a 
5-ton automobile truck and distributed in piles about 30 feet apart 
along the road. The spreading was done by hand. 
EXPERIMENT No. 1.—SurRFAcE TREATMENT WITH COAL-TAR PREPARATION, APPLIED 
Cop. 
Location, station 0+00 to station 23-+-55. 
Total length, 2,355 feet. 
Total area, 4,187 square yards. 
The tar used in this experiment was hauled from Washington 
in the auto distributor, and applied at the average rate of 0.49 
gallon per square yard. Table 27 shows the analysis of the tar. The 
temperature of the material at the time it was applied was about 
150CcR. 
