One quick and dirty way to break out the numbers would be to create a danger number, which would be 100 minus the 1 to 100 integrity scale.  This could be the formula: danger factor = daily volume * (100 - bridge integrity rating).  So a perfect bridge with 300,000 cars a day would rate a 0.  A bridge with a 75 integrity rating and 10,000 cars would get a 2,500 score, and the I-35W bridge, with 141,000 vehicles per day and a 50 integrityh rating would get a danger score of 70,500.  I'm sure the I-35W bridge would nearly top this list that accounts for vehicle volume and structural integrity.