Then they’d actually be legitimate road vehicles and share with all the other ones instead of having part of the road taxes siphoned off for a special lane that’s empty most of the time.
If you’re expecting them to contribute equally, than they need an equal share of the road, or else cars would bully them onto,the side of the road. It would, effectively, cause them to pay a disproportionate share of taxes, as they would pay an equal amount of road tax, but not get an equal share of the road.