Climate Progress provides a good explanation of the impact the recent Supreme Court ruling will have on states that want to limit auto emmissions.
How does this translate into action? Well, it doesn’t necessarily. It means that states wanting automobile emission standards more strict than those set by the federal government are allowed to pursue them. So it removes an obstacle. They have to receive a waiver from the EPA, but are now more likely to be granted one. States do not have to act, but ones that want to, will (and have).The ruling removes one roadblock from localities using the EPA to support their own determinations of how their environments can be better protected. As Climate Progress puts it, "Slowly but surely, the climate is finally manuveuring its way through Washington, DC."