26 
BULLETIN 1486, U. S. DEPAETMENT OF AGRICULTURE 
structure. If the P. C. of horizontal curvature falls on the structure, 
full superelevation is not obtained at the P. C. If the P. C. falls just 
outside the structure, the superelevation at the end of the bridge 
is less than the superelevation which will be obtained at the P. C. 
If the P. C. falls within the limits of the structure and if full super- 
elevation must be furnished at this point and carried along the 
remaining portion of the structure at this same value, a special 
treatment will be necessary. 
For bridges lying adjacent to, or partially on horizontal curves at 
either end, having opposite directions, a treatment, as shown in Figure 
24 B, should be adopted, each rail being given a smooth continuous 
curve from end to end. For structures having curves at either end 
in the same direction, full superelevation should be carried across 
the bridge, the rails being treated as separate parallel curves. 
UPPER RAIL 
LOWER RAIL 
RIGHT RAIL 
O 1 
LEFT RAIL 
/ 
B 
Fig. 24. — A shows an improper method of rail treatment in superelevating the roadway which 
gives an appearance of sag. B shows a correct treatment of rails where the bridge is either on or 
adjacent to curvature in opposite directions at the ends 
TRAFFIC INFLUENCE ON BRIDGE LOCATION 
It may be stated as a basic principle, that the location of any bridge 
structure should be such as to render the maximum service to traffic 
at minimum cost. Cost considerations have already been discussed 
in this connection but certain traffic influences controlling the selec- 
tion of site should be mentioned. 
A highway bridge, to give the best possible traffic service, must be 
located at the best possible routing point, and in this connection it, 
may be well to consider separately three principal types of traffic 
through, regional, and local. 
THROUGH TRAFFIC CONSIDERATIONS 
Under this head should be listed considerations involving the 
following : 
National defense. 
Movement of troops and munitions. 
Safety from enemy attack. 
Ability to protect bridgeheads. 
