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 503ms−1 to 2009ms−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 _______________.