National guidelines call for at least a 3.2-second yellow light in 30 mph zones. But national experts consulted by the Tribune said prevailing speeds — not the posted speeds — should be used in setting yellow light times and that Chicago's own data on prevailing speeds means yellow lights should be lengthened to at least 3.5 seconds on its 30 mph streets. Research indicates that such a change could bring down rear-end crashes caused by drivers slamming on their brakes, while also reducing red light camera tickets that have generated more than half a billion dollars for the city.