This disclosure provides devices, computer programs and methods for determining a motion direction. In one aspect, a mobile device includes sensors for measuring acceleration data. The mobile device also includes a processor and a memory that implement a motion direction estimation module configured to determine a primary axis of motion. The motion direction estimation module also determines a motion direction along the primary axis. The determination includes fitting the acceleration data, or data derived therefrom, to a bimodal distribution. A first peak of the bimodal distribution corresponds to a first motion direction along the primary axis, and a second peak corresponds to a second motion direction opposite the first. The motion direction estimation module is configured to estimate the motion direction based on the bimodal distribution. In some implementations, the motion direction estimation module selects the motion direction corresponding to the higher of the peaks as the estimated motion direction.