Before revision: doi:10.1016/0191-2615(94)90002-7 Relative to a fixed spot on the road the wave can move with, or against the traffic, or even be stationary (when the waves moves away from the traffic with exactly the same speed as the traffic).
After revision: doi:10.1016/0191-2615(94)90002-7 Relative to a fixed spot on the road the wave can move with, or against the traffic, or even be stationary (when the wave moves away from the traffic with exactly the same speed as the traffic).