The Clean Air Act went into effect stateside with 1968MY vehicles, hence 52 years of vehicle emissions limits imposed. Every few years we have been forced to take a bigger leap toward zero-emissions, hence a graph of emissions vs time showing a negative quadratic (an "upside-down" parabola as some students say). If you follow the trend, in the next few years we will have engines producing negative (less than zero) emissions.
Future implications of that example aside, the trend itself is simply illogical, hence why its a favorite example in discussion of the ivory-tower govt/academic crowd.
Rethink that point a minute, it doesn't make sense. If you're against children running around in smog then the logical conclusion would've been to require large reductions in emissions 50 years ago to bring those levels down asap, not slowly ramp them down as we did. y = 1/x makes more sense than y = -(x^2)+b.
I'm not suggesting the goal was wrong bc political/social goals usually aren't IMHO, I'm just saying that the method of achieving those goals needs to be logical and created by the industry implementing them. Most technology develops contrary to Moore's law - early developments tend to be the largest and over time we hit physics-based limits and the speed of development/progress generally slows over time. In this case, common sense says that big reductions in emissions should've been undertaken ~50 years ago when the "reduction" was fairly easy tasks like reducing vehicle weight (shrinking land yachts) and switching to fuel injection rather than trying to make huge leaps based on minor tweaks to improve efficiency as we are today.
