Bridge Engineering Fundamentals The clearance of a bridge span is typically measured from the water's surface (or the ground if there is no water) to the bridge's undersurface. Enough clearance should be built into the bridge design to assure traffic safety beneath it. Bridges are usually designed with enough space for a vehicle to pass under them, but this distance can be increased or decreased by adding or removing material from either side of the bridge.
The minimum clearance required between a bridge and an approaching vehicle is called its "minimal clear width". This amount varies depending on how close the vehicle gets to the bridge while still being safe under normal conditions. If the vehicle is driven fast, over or off-road, or in some other way put at risk, then more clearance is needed. Clearance requirements are usually listed in regulations issued by state agencies that oversee bridges.
For example, most bridges across streams or rivers in their active seasons (spring runoff and/or fall flooding) require a vehicle to have at least two feet of clear space underneath it. This ensures that vehicles won't be hit if a branch, twig, rock, etc. is lying on the riverbed. Some states require three feet of clear space to ensure adequate driver awareness.
Bridges also need to be wide enough to allow fire trucks to drive under them.
Standards Horizontal clearance beneath or along a bridge must equal the entire paved width of the road. Bridges over 200 feet (61 m) in length may be narrower, with a minimum of 4 feet (1.2 m) on both sides of the vehicle lanes. Clearance below bridges is usually 8 feet (2.4 m), except where pavement has been worn away by traffic down to the base of the support columns.
Longitudinal clearance between the side of a vehicle and the edge of a bridge is called "side-clearance." It should be enough room for a driver to get his or her arm out the window without hitting anything while driving at normal speeds. Side-clearance varies depending on how far back the first row of seats is pulled up toward the ceiling. The farther back the seat, the less space there is between the side of the car and the bridge.
The distance from the outermost point on one rail to the corresponding point on the other rail is called the "gauge" of the bridge. Most railroad bridges have a standard gauge of four feet (1.2 m). Some older bridges had a gauge as low as three feet (1 m), while others are eight feet (2.4 m) wide. A bridge that does not fit onto a standard-gauge track cannot be used by trains traveling at conventional speeds.
The vertical clearance of an overpass bridge is defined as the minimum height between the pavement and the bottom of the overpass structure and must be at least 16 feet throughout the whole width of the highway, including the auxiliary lanes and the paved shoulder. The clearance should be measured from the top of the outermost surface of the vehicle to the bottom of the overpass structure.
The distance between the tops of adjacent traffic signals or warning devices is required to be a minimum of 10 feet for any type of roadway. This means that you cannot put a signal next to another signal unless there is at least 10 feet between them. Traffic signals can be placed on raised platforms or poles if they do not exceed 6 feet in height. If the signals are mounted on such structures then the clearance needed under these signals is also 6 feet.
The clearance required under a freeway overpass is usually less than that required under an ordinary road-grade overpass because the undercarriage of a truck is typically higher than that of a car. The requirement for clear underpass/overpass height applies to all vehicles, regardless of size. However only large trucks need additional clearance under a freeway overpass since their undercarriages are high enough to clear the underside of the bridge.
Freeway overpasses come in three main types: conventional, modular, and cable-stayed.