in rainy days, flowing water in river is so high and very dangerous. that's why for safety,bridges are built so high by engineers.
It is to avoid having to build a drawbridge to allow ship or barge traffic to pass below. Some ships are taller than a 14 story building. (By the way, the aircraft carrier, USS Enterprise can only pass under the Golden Gate Bridge at low tide and by removing some of its antennas). That is a greater hazard to a truck or vehicular traffic if the driver doesn't stop in time and plunges into the water below when the drawbridge is up. Swing and lift bridges were also used at one time. It is also a convenience to avoid obstructing, delaying, and reducing the flow of vehicular traffic.
See also the new highway bridge that bypasses the Hoover Dam that saves hours of travel time through the canyon.
Generally a bridge is built to allow passage of traffic below it. If it is over a waterway, it needs to be high enough to allow ships to pass underneath.
Or there may be other existing geographic features that drive a given design.
it depends upon :
- connection streets position
- ships to pass beneath
sometimes a bridge will be above a lake or it will just be above a grass field. why are bridges built 120 ft high? it should be considered more of a hazard to drivers
