Carbon Emission Moment

I saw a recent article that claimed that traffic at roundabouts caused 42% less carbon emissions than traffic waiting at traffic lights. Imagine a simple white line was painted across the lanes on busy intersections, perhaps 10 car lengths back from the traffic lights. Imagine that any car after that line was obliged to shut off its engine until the light was green. Could this help reduce carbon emissions at least within city limits? A simple idea. A bit of white paint.