An interesting statistic from the US Department of Transportation:
"2000 vehicles per hour sound twice as loud as 200 vehicles per hour".
In other words, if you start with an empty road, the first cars that you
"add" to this road contribute much more to perceived noise than additional
cars. Or, alternately, the last cars that you would remove from a road are
responsible for a disproportionate share of the noise reduction.
And "one truck sounds as loud as 28 cars"!