Well the issue is, that statement isn't true.
Using your example, dependent on speed and other variables, doing a blast in 20 minutes will produce more than double the emissions than your neighbour doing his journey in 40 minutes at a steady lower speed. There's lab research that proves at 60mph travelling for a few minutes more still reduces emissions significantly compared to travelling at 70mph for a shorter period of time. In reality, the difference is probably even more as we all know most vehicles are blasting down the motorway at 80mph+.
The problem with the 60mph rules is that it doesnt make any logical sense. If it was about "Air quality" then EV's should be exempt. Maybe its the cynic in me but I reckon its probably a good money maker.