The way it works in some places is that road tax is funded from taxes from general population plus road tax for cars, trucks and motorcycles plus, for highways, extra taxes for cars and trucks. The car use of pedestrians and bicycles has minimal to no impact on maintenance (I know sidewalks built more than 50 years ago that never needed any maintenance), while the part of the road where cars drive is impacted by the cars and a lot more by trucks, so the system above seems quite fair. Also highway taxes funded only by cars and trucks is very fair, pedestrians or cyclists are not allowed there.