The US gas tax is essentially a "user fee" for the highway system. Revenues from the federal tax go into the "highway trust fund" and every five years (or whenever congress gets off its lazy butt) congress passes a "highway bill" (now known by cool acronyms like ISETEA, TEA-21, SAFETEA-LU) which allocates those funds to State DOTs and pork-barrel projects (like the famous "bridge to nowhere" in Alaska).

The point being that the US gas tax was not intended for environmental purposes or to change behavior. It's a way to fund transportation projects.

Maybe the european taxes had a different purpose, or european voters have accepted a gradual change in scope more willingly than US voters.