Abstract: A formula for determining the propagation speed oftraffic waves in real traffic flows is derived based on the physicalmechanism of car-following, which can be used to calculate the starting wavespeed at the urban signalized intersections as the red light turns to greenone and the incident wave speed on expressways. It is proved that thepresented formula is reasonable and applicable through the comparisonbetween the calculated results and in situ measured data. In addition, thecharacters of traffic wave speed, especially expressway incident wave speed,are discussed and consistent with daily knowledge in realistic traffic.