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.