YOu're overthinking it. It's about driveability If the rear wheels ain't turning, you should turn on the emission controls. When the car is stopped in traffic, might as well make it clean. Performance isn't an issue when stopped. I'd have taken it a step further and made it clean whenever driveability isn't compromised...like when not accelerating at a rate faster than you could do with the emission controls functioning. Probably would never have been detected.