Architectural drawing requires a level of precision that can prove maddening if attempted entirely in freehand. Formulaic techniques keep drawings in correct perspective, all which involve vanishing points. Perspective can be in one-point, where objects are viewed from directly in front, or two or three-point perspective, where the angle of observation is skewed to varying degrees. Regardless of subject matter -- bridges, highways, swimming pools, or hallways -- the receding lines of any object can be traced to a vanishing point on the horizon line.
Draw a horizontal line across the entire sheet of paper, about halfway between the top and bottom of the sheet. This is your horizon line.
Draw a dot on the horizon line, preferably near the middle of the paper. This point is known as the vanishing point.
Draw two equal-length, vertical uprights, one on each side of the dot. These will serve as the near end of the bridge, so space them accordingly.
Connect the bottoms of the two vertical lines with a single horizontal line.
Draw two lines: one from the dot to the bottom of one upright, then a second line from the dot to the other upright. These lines form the level of the road surface along the bridge.
Draw two more lines: one from the dot to the top of one of the uprights, and then the second to the top of the other. These lines form the uppermost level of the suspensions or other upper supports.
Draw two final parallel vertical lines near the vanishing point, each one running from a point along the upper-support line to the surface line. These form the far end of the bridge.
Connect the bottoms of these two vertical lines with a single horizontal line.
Erase the upper and lower lines from the vanishing point to the far end of the bridge.
Use the T-square for every straight line, and to ensure that your angles are square. In one-point perspective, all straight lines will either be horizontal, vertical, or extending from the point closest to the foreground toward the vanishing point. When working with perspective, exactness matters. Keep your pencil sharp and use a straight edge long enough to draw each line with a single stroke.
Light lines erase easier than heavy lines. Don't press too hard on your pencil when drafting your initial lines.