20 BULLETIN I486. TT. S. DEPARTMENT OF AGEICLXTTBE 
Navigation requirements have an important effect on the minimum 
vertical clearance which may be adopted and this in turn vitally 
affects the location. It is not possible to provide against runaway 
boats and ships but in many cases clearance for barges through not 
only the closed movable span, but also imder the adjacentf fixed 
spans may be provided at but slightly increased cost by a proper 
location. 
ALIGNMENT CONSIDERATIONS 
A highway location, especially through rough country, must paral- 
lel waterway and ridge lines for a major portion of its length. A 
crossing of a waterway, canyon, or ravine, therefore, generally 
involves an approach alignment down one side of the canyon, crossing 
the stream or canyon and up on the other side, either doubling back 
or proceeding ahead. If the line after crossing the canyon doubles 
back upon itself to gain distance, it is in many cases impossible to 
eliminate cruvature from the bridge approaches except at a prohi- 
bitive expense. If, on the other hand, the location proceeds across 
the canyon and on in the same general direction, some of the curva- 
ture may be eliminated by the selection of a skew crossing. 
Curvature is particularly to be avoided on the approaches of 
bridge structures because of certain structural difficulties involved. 
With the high-speed motor vehicle traffic most of the States have for 
some time superelevated curves as standard practice and lack of 
superelevation is considered an element of danger. A superelevated 
bridge approach involves the construction of hand railings, which 
must be either out of plumb or else not perpendicular to the deck. 
If such railings are placed perpendicular to a superelevated deck, 
thus being canted out of the vertical, it gives an appearance of sag 
and incorrect alignment. If, on the other hand, the railings are 
placed in a vertical plane, the inner rail obstructs the roadway 
clearance by an amount equal to the height of the railing multiplied 
by the sine of the angle of superelevation. This clearance restriction 
is. of course, very small, but may be just sufficient to expose the rail 
cap to impact from vehicles. Cases have been observed where 
concrete railings of this type are seriously marred, chipped and broken, 
owing to the above cause. (See pi. 5. A.) The problem involves the 
balancing of approach curvature with its undesirable details against 
the additional expense and general undesirability of a skewed cross- 
ing. If the skewed crossing is particularly undesirable because of 
stream conditions, or if the location is such that skewing the bridge 
will not reduce approach curvature, it may be necessary to consider 
throwing the approach alignment on either side back into the hill 
in order to obtain a requisite tangent length for the structure. This 
procedure generally involves an increase in the cost of the line and 
in many cases necessitates increasing the rate of curvature away 
from the bridge. 
On many bridge crossings avoiding curvature is a difficult matter 
and involves a substantial increase in the cost of the line. In such 
cases considerable tune may be well spent in careful and painstaking 
preliminary study. The location is perhaps the most permanent part 
of a highway system. 
For a bridge of any size it should be considered that the location is 
to be fixed for all time to come and that if any error is made the 
