28 
BULLETIN I486, U. S. DEPARTMENT OP AGRICULTURE 
160 feet and for the largest naval craft perhaps even a greater clear- 
ance. Ordinary river traffic can generally be accommodated by a 
vertical clearance of from 60 to 80 feet above maximum boating stage. 
If a true high-level location (employing fixed spans) is not feasible, 
a structure on a grade such that openings are required only for the 
highest masts will be more desirable in that the greater portion of 
the craft using the waterway can pass under the closed channel span 
or perhaps under adjacent fixed spans without the delay incident to 
" standing off" for the draw to open. 
resort 
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Fig. 25.— Desirable bridge locations under various traffic conditions adjacent to a congested traffic district 
From the standpoint of land traffic, a movable span will generally 
involve shorter approaches, a more direct connection with industries 
located on the river front and less grade. On the other hand, the delay 
to land traffic due to closure of the roadway during the operation of 
the movable span will generally be a considerable item. Some of the 
bascule spans in the city of Chicago open an average of seventy-five 
times per day. At a conservative estimate, this closes the crossing 
for three hours during the day. Assuming the bulk of traffic as being 
t 
