They are a massive transfer to public land to whoever occupies the road, and the person occupying the road might not even be in their steel box for days on end.
Well, it depends on how the roads are financed. You are right that roads financed out of general taxation and free to use can be seen as direct or indirect subsidies to the car industry.
But the same physical road, but financed out of user-fees (or by voluntary contributions from nearby shops to attract shoppers etc) by a profit-driven private company, are not subsidies to the car industry.
Or take the hypothetical from the last paragraph, and add massive taxes on top, and all of a sudden it's the opposite of a subsidy. But the physical road stays the same.
They are a subsidy to the car industry.
They require ongoing maintenance.
They are a massive transfer to public land to whoever occupies the road, and the person occupying the road might not even be in their steel box for days on end.