A system estimates the speed of a moving vehicle and hence the driving behavior of an individual driving the vehicle using accelerometer data. To do so, the system analyzes received accelerometer data to find idling points when the vehicle is not moving during a driving session. Based on the idling points, the system may divide the driving session into two or more segments. The system may then determine the speed of the vehicle at one or more boundary points of each segment. For each segment, the system may analyze the accelerometer data to determine the acceleration of the vehicle for points when the vehicle is moving. Subsequently, the system may calculate the speed of the vehicle for the points when the vehicle is moving based on the acceleration of the vehicle at the points when the vehicle is moving and the speed of the vehicle at the boundary points.