Anisotropic property revisited--does it hold in multi-lane traffic?

The anisotropic property of traffic flow has brought much discussion about some existing traffic flow models and diverging views on the further development of traffic flow theories. This paper shows that the anisotropic property cannot be expected to hold in multi-lane traffic. Moreover, it develops a mathematical criterion to judge the anisotropic property of a traffic flow model, shows how non-anisotropic behavior arises from multi-lane traffic, and why some higher-order models do not always behave anisotropically.