Abstract: Evaluating bridges using field tests is a useful method for determining a structures in-situ performance. Actual bridge performance determined in this manner has typically been found to be better than that predicted based on analytical models. In this paper, a method of obtaining influence lines for short-to-medium span bridges directly from truck load tests is presented. These “experimental influence lines” are compared to influence lines developed based on analytical methods, or simplified load distribution assumptions in order to quantify differences between the actual and assumed bridge performance. The proposed methodology is demonstrated using results of a recent load test conducted on the South Market Street Bridge in Wilmington, Delaware.