Making them toll roads (or charging congestion fees at peak hours) would fix individual incentives and help with general traffic issues. Existing gas taxes are

Making them toll roads (or charging congestion fees at peak hours) would fix individual incentives and help with general traffic issues. Existing gas taxes are a halfway-decent proxy, but that's becoming less true as more adopt electrics and hybrids. Of course, it's important that fees apply to all vehicles. When London implemented a congestion charge, taxis were exempt. It wasn't long before people noticed that minicab registration cost much less than a year of congestion charges. The loophole has been fixed, but it's still an amusing example of incentives.