Design and evaluation of prestressed concrete I-girder bridges are dependent on the service level, live load, tensile stresses induced in the girders and the distribution of these stresses among the supporting girders. This study presents the results of field tests conducted on three prestressed concrete I-girder bridges to obtain service level stresses and girder distribution factors. The three bridges are simple span structures with span lengths of 10.3 m, 23.3 m and 31.1 m. Bridge live load response was measured near the mid-span for the passage of test trucks and normal truck traffic. The study found that measured service level stresses extrapolated to a recurrence interval of 2 weeks to 75 years exceed levels induced by an HS-20 truck in interior girders only. Measured girder distribution factors for one and two-lanes are less than girder distribution factors specified by the American Association of State Highway and Transportation Officials.