Maybe a tax on tires then? Yeah, that's it! Tires. Auto tires, truck tires, bike tires, scooter tires. A tire tax would address the portion of vehicles that actually contact the roadways. It would be fair because, for instance, a unicyclist would pay tax on only one tire, a bicyclist pays tax on two, etc. The tax increases incrementally to the 18-wheelers. 
It's perfect, IMO. 
I will contact Salem tomorrow.