Driver's arm viscoelastic properties during steering vehicle maneuver are very important information to understand how the driver regulates his torque and they could be used to design a new driver assistance system. This correspondence paper investigates the driver's arm viscoelastic properties during steering vehicle maneuver. First, a time-varying method for estimating driver's arm viscoelastic properties during driving the vehicle is proposed. Then, this method is implemented experimentally in order to examine the driver's arm viscoelastic properties during driving the vehicle. The estimation method does not require a specific perturbation torque to be applied during driving and could be applied online. The method is based on a time-varying model of the human driver's arm coupled with the electric power-assisted steering system model. Experiments were carried out with five healthy subjects who drove on a hardware-in-the-loop simulator without assistance and with assistance.