"This motorway" is not "az autópályán "
You're right. It means "Ezen az autópályán".
Why this ambiguous wording? One does not know where the road goes? I cannot say that this sentence is grammatically incorrect, but I would never say this in English.
exactly. It would be either move towards lake balaton or move in that/this direction
Perhaps more elegant English would be: 'The cars on the motorway move in the direction of Lake Balaton'.
In other words, 'in some direction towards' = 'in the direction of'