Freeway Maniac, The

It will turn on you and the flashing lights and
Pivoting from street to street, stop traffic as manic.
As a shield to do without it ...
Yes it is on the highway ... Does anyone know why?

This is called a moving roadblock. The exact reason is not really possible to know, but it's usually because they need to keep traffic away from anything before, like a police chase, motorcade or another, etc.

Yes it is a roadblock to slow passing traffic for any reason. Perhaps resentment accident, dangerous objects on the road to be allowed, etc etc..