[aerohead]

It appears to be completely 'relative', as some don't consider aerodynamic drag as something which would inform their thinking or behavior at all.
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As CAFE standards have increased over the decades, along with frontal projected areas, there's been 'some' interest in aerodynamic drag, associated with regulatory compliance by automakers, and some nascent awakening as to the relationship between the climate and road vehicle aero drag ( global warming was mentioned in my 1974 college text for internal combustion engines ).
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As to 'numbers', they've always lurked within the mathematics of the 'force' and 'power' equations as related to 'velocity'.
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It has been accepted for some time now, that, one might consider aerodynamic drag at as low as 35-mph ( 56-km/h ).
At 70-mph, the drag force would be 4X what it is at 35-mph ( velocity-squared law )
At 70-mph, the horsepower / Kilowatts necessary to overcome the drag would be 8X what it is at 35-mph ( the velocity-cubed-law ) [ you're hitting twice as much air, twice as often, and twice as hard ].
-----------------------------------------------------------------------------------Aerodynamically, you could manufacture an automobile today, which would get the same mpg at 100-mph ( 161-km/h ), as it presently gets at 55-mph ( 88.5-km/h ).
If you drove 'that' car at 55-mph, it would resemble driving on the Moon.