What is your opinion of this traffic law and auto mufflers?

There is a traffic law in my state, that states a vehicle’s muffler shall be installed as to not allow excessive noise or smoke from the automobiles engine. I have always thought this law was not enforceable, since a cars muffler cannot control the amount of smoke emitted by the cars engine. I am surprised that some smart attorney did not challenge this in court, but it never happened. Question: what were the lawmakers thinking, when they wrote and passed this law? I can understand the noise portion, but not the smoke portion. Opinions?