Rules of perspective recommendedYou're trying to make a neat use of perspective with this bridge gapping into distance. But iy looks unreal because you do not make use of perspective. Allow me to explain you the basic rules of perspective with this image:
http://img260.imageshack.us/img260/9606/1ptperspectivewb3.pngThe Horizon, is also your eye level. The VP is a vanishing point, this is a 1 point perspective image. In the top left I make an outline for the bridge structure, this version having 2 pillars and 1 butress inbetween. Now I drag it all the way towards the horizon, to the vanishing point. This is because the thickness of this object will be bent towards perspective. Also, because the other side of the bridge is positioned linear to this side, we can draw the other piece somwhere along the help lines towards the vanishing point, we can choose for ourselves where because we have no set length of the bridge.
Inbetween I just used the curved line tool of MS paint and kinda guessed where the center of the bridge would be within this perspective. You should aso know that in perspective the distance seems to get less and less, therefore the density of the lines representing the cables should get higher and higher, I didnt pay high attention to it, but if you've got a photo of a street with lots of streetlights, you can see for yourself and estimate what will look convincing in your image.
Lets keep it at this, even though fully realistic images have 3 points in perspective, you can learn about 2 point perspective later on, there are some good methods to create 2 point perspective images using a plan view and some height measurements.