It's not useless: a reduction in 4-5% of emissions in urban stop and go traffic would be huge for air quality and health. It likely won't do much if anything for total CO2 output but offloading the emissions to manufacturing of parts needed for extra maintenance (aka a far more efficient power plant in a less densely populated area) instead of driving can still be a net win.

Regardless, fuel efficiency standards have driven a significant amount of innovation even if this specific rule seems absurd from the outside.