Anyway, back on topic...
This is where it get's way too convoluted. I'll use an extreme example to help. Say, you ride a motorcyle round trip to work each day. I drive a big rig. Or, I'll even say I drive a Hummer. Who do you think damages the roads more over time? And what do you charge companies who use massive amounts of tranportation, ie; big rigs, etc? Do certain roads cost less than others? Should they? What if a portion of your drive is on a dirt road or a city/municipal, etc road? In this case, you have to worry about paying your milage for driving your car while not using a road that is supposed to be covered under this tax. On top this, you have to add in all the extra equipment to monitor milage, development of said equipment, installation, training for installers, training and hiring for monitors, possible factory line alteration for installation, new bills/laws passed concerning equipment, manufacturing of, privacy issues, tampering with, etc.
Anyway, I could go a whole 'nother paragraph. But you get the point and it gets you on track.