Let's suppose that u sees u' moving east at, say, 10 m/s. If they are next to one another at t= 0 then u will see u' move 100 m in 10 seconds. From u' point of view, u is moving west but he will still measure 100 m between them after 10 seconds. That's why u' sees u moving at 10m/s also but in the opposite direction: [itex]v_u= v_{u'}[/itex]
