I know how you’ve calculated maximum number of cars and I’ve told you that’s where the flaw is.
Say 10,000 cars travel between J33 and J34 per day. Using your units;
At 60 mph, it’s 60 x 10,000 pollutant seconds per mile.
At 70 mph it’s 51.42 x 10,000 pollutant seconds per mile.
60/51.42 = 1.167.
The “maximum number of cars” you are using is entirely irrelevant. Your analysis isn’t simplistic it’s just wrong. The number of cars that have to get between J33 and J34 does not change regardless of how fast they do it.