The objectives of this project were to develop an analytical tool and to use it to evaluate the relative user responsibilities for fatigue damage to highway bridges. This tool was designed to be a general analytical tool to evaluate the current fleet mix database established by FHWA. It is a FORTRAN program designed to read standard format FHWA files for vehicle data and vehicle kilometers traveled. The output is a relative fatigue damage matrix for each vehicle class and weight group over 12 FHWA-established functional highway classes. The relative damage factors are then used as input into the highway cost allocation study to assign costs of fatigue damage. This study advances the analytical technique for evaluation of relative fatigue damage to highway bridges by incorporating the semicontinuum analysis method, the Palmgren-Miner hypothesis, and the rain-flow cycle counting algorithm. The analysis evaluates a database of 39 bridges selected as representative of those in the United States and contains several hundred fatigue-prone details for analysis. The vehicle database was developed by FHWA specifically for the highway cost allocation study. Reviewed in this paper are the basis of the fatigue analysis used for this study and all assumptions made in the development of the fatigue program. The vehicle database and bridge selection and database incorporated into this study are also described. Results of the analysis are summarized and presented.