the left of the circle is the overhead clearance and to the right is the overall bridge length. This type of
signing is normally only posted on major double-flow bridges.
In many cases, bridges will be marked with standardized circular bridge signs. These are yellow circles
with black numbers. There are two types of such signs, normal and special. The engineers determine
what type of sign will be used.
All bridges not having the full NATO sign should have one of the circular signs. Normal signs include
those for one-way bridges and those for two-way bridges. One-way bridges will have the bridge
classification in black numerals on the yellow circle. These are normally found on bridges less than
class 50. A two-way bridge sign will be divided in half vertically. On the left side half of the circle will be
the two-way classification and on the right, the one-way. In certain circumstances, the engineers may
decide to use special signs. This is often the case in bridges over class 50, since those may be used
by the larger tracked vehicles.
In those cases, a separate classification is indicated for wheeled and tracked vehicles. For a one-way
bridge, the circle will be divided horizontally. The upper half will tell you the wheeled classification and
the lower the tracked. They normally contain a silhouette of a wheeled and tracked vehicle,
respectively, to prevent confusion. In some cases, it may be desirable to show different classifications
for wheeled and tracked vehicles on a two-way bridge. This is done by using two circles, one above
the other. The top symbol provides the information for wheeled vehicles and the lower for tracked.
You have spent a great deal of time dealing with bridge symbols. This is because they are normally the
weakest link in the route. Therefore, in the route classification formula, the lowest bridge classification
'number is entered. This is regardless of the vehicle type or condition of traffic flow. The smallest
number provides the classification for the entire route. Only in very rare and unusual circumstances will
this not be the case. The most common of these circumstances is when there are no bridges on the
route. In that case, the worst section of the road sets the route classification.
Overhead Clearance. The next entry in the route classification formula indicates the overhead
clearance. The overhead clearance is the distance between the road surface and any obstruction
above it that restricts traffic flow. It is the clearance that you often see displayed on bridges and
overpasses. If the clearance on the route is unlimited, the symbol for infinity is used. Overhead
clearance is considered an obstruction if it is less than 4.3 meters (14 feet). Whatever the overhead
clearance, it is noted. As with the other categories of information, the lowest clearance on the route is
the one which is listed.
Obstructions. If there are any obstructions on the route, they are indicated in the formula by the symbol
"OB". Anything that restricts the type, amount, or speed of the traffic flow is considered an obstruction.
The only exception