Effects on in-service pavements from superheavy overloads exceeding 9 340 kN were investigated by field study and mechanistic modeling. Field study consisted of conducting a crack survey, measuring pavement deflections using a Dynaflect before and after hauling the overloads, and using three nondestructive testing (NDT) instruments to measure surface deflection when the overloads were hauled. In addition, field study included measuring static wheel loads and extracting cores. Prior to hauling, deflections were predicted using elastic layer computer models for assumed “best” and “worst” case scenarios. These initial predictions are compared to measured in-transit seismometer deflections. Layer moduli later were backcalculated using the BISDEF computer model and Dynaflect data. Results of crack surveys show no change in visible condition of the pavement after transporting the overloads. Dynaflect measurements after transport were approximately equal to pavement deflections measured before hauling the overloads. In-transit deflection measurements show that a “big basin” results from widely distributed trailer axle loads. Deflections from tractor tires were not substantially different from those caused by trailer tires. Measured in-transit deflection is less than the “best” and “worst”case initial predictions. Subsequent backcalculation yielded a wide range of layer moduli. However, these moduli resulted in final estimated deflections that are much closer than initial predictions to field measurements.