How do coal rollers control when their vehicle spews black smoke and when it does not?

[Machine_Elf]

States don’t usually check individual vehicle emissions unless they have a problem meeting NAAQS:

National Ambient Air Quality Standards

If a state or region is found to be in violation of EPA-mandated NAAQS, then they’re required to develop a SIP:

Basic Information about Air Quality SIPs | US EPA

A SIP is a state’s plan for achieving NAAQS compliance and often targets privately owned vehicles, either with an annual smog check requirement, or (as Denver does) a drive-by roadside emissions check.

How does this drive-by emissions test work? (Northern Virginia)

The typical location is a highway on-ramp where you are accelerating, i.e. you’re making a significant amount of power (and therefore a useful amount of exhaust). The measurement is optical: the system is sending a light beam across the roadway, roughly through the region where your car will be spewing exhaust, and a receiver on the far side of the road measures the attenuation of specific wavelengths of the beam by the components of your exhaust. The higher the concentration of criteria pol…

If a state is in NAAQS compliance, then they often aren’t equipped to do emissions checks. As noted upthread though, the EPA is targeting bigger fish, with some success:

EPA says Windham dealership tampered with truck emissions controls

Clean Air Act Vehicle and Engine Enforcement Case Resolutions | US EPA