I think some form of limit aware speed chime or warning is required on new cars sold in the EU. Having driven a few, I would not want to trust that technology to slow the car down.

Many times I have had a car’s camera pick up a speed limit sign from a neighbouring road to the highway and indicate that I was suddenly going 60 km/h over the new speed limit. Same thing if GPS tracking or map data isn’t perfect on cars that use that instead.

I wonder if it’s not so much the density/opacity, but rather refraction. Significant difference in density meaning diffraction, and we’re seeing the comparatively dark sky refracted through the exhaust plume?