thats a strangely abstract way of looking at it. i think of it as a way to capture some of the externalities of car use, the environmental, social and public health damage.
The reality is probably that it's not that well coupled to any cause, and really it's just a way to farm money from the majority of the country, who need to use motor transport to live their lives. The advertised political reason will change depending on the concerns of the populace at the time.