In the West Point Bridge Designer 2010, an optimum design is one that costs the least. After you have considered as many different alternative site and truss configurations as possible, choose the one with the lowest total cost as your final design.
In the design of an actual bridge, there are many other criteria that would also be taken into account in selecting the most desirable alternative. These include aesthetics, ease of construction, ease of maintenance, local availability of materials, and environmental impact.