Which of the following statements CANNOT be used to describe free flow speed $(u_f)$ of a traffic stream?

1. $(u_f)$ is the speed when flow is negligible
2. $(u_f)$ is the speed when density is negligible
3. $(u_f)$ is affected by geometry and surface conditions of the road
4. $(u_f)$ is the speed at which flow is maximum and density is optimum