A modified two-dimensional (2D) Moses algorithm for acquiring the field-calibrated influence line (IL) of an existing bridge is presented, based on strain data acquired continuously at a high scanning rate with calibration vehicles of known axle weights and axle spacings crossing an instrumented bridge. Considering the transverse distribution of the wheel loads on each girder attributable to the 2D behavior of the slab-girder bridge, the IL of each of the girders can be calculated, which does not require the girders to possess identical material and geometric properties. By using the calculated IL of each girder as a reference, a modified 2D Moses algorithm was derived to identify axle weights of moving vehicles, taking into consideration the transverse distribution of the wheel loads on each girder. Mathematical equations to calculate ILs and axle weights were derived, and the proposed algorithms were implemented by a computer program. The accuracy of the IL calculation and axle weight identification was verified through a field test of a bridge on U.S. Route 78 in Alabama. The identified axle weights showed agreement with the static measurements from weighing pads and with results from the bending-plate weigh-in-motion (BPWIM) system near the instrumented bridge. © 2013 American Society of Civil Engineers.