Wow, always putting earplugs in when you drive. Don't get me wrong, it is smart, just I lack the discipline to do it every time.
Plus, I've never been one to have ear infections, so I end up with used ones in all my pockets so they're pretty much always on me.
As another comment mentioned, there are flat frequency response ear plugs, Etymotic ETY ER20 are pretty cheap, reusable and cleanable. Not as much dB reduction, but that's better for some scenarios.
And a car would have pretty decent internal damping.
The general traffic is already way too loud. Can reach 80 dBA and even more in tunnels.