On a city road, the last traffic light glows green for 60 s and red for 120 s. The range of speeds of vehicles is from $\frac{50}{3}{\mathrm{ms}}^{-1}\text{ to }\frac{200}{9}{\mathrm{ms}}^{-1}$ in a group. The speed of each  vehicle is constant. It is found that at a distance x from traffic light, then the successive groups pushing through traffic light place will become indistinguishable. The value of x (in km) is _______________.