Have you ever been stuck in highway traffic for hours before suddenly emerging into free-flowing traffic, with no sign of anything that caused the disruption? Growing numbers of scientists are exploring what some researchers call "jamitons," or phantom traffic jams. Scientists are learning that under the right conditions, even a slight change — one person putting on the brakes, for example — can cause a massive traffic jam that lasts for hours. "Jamitons occur when traffic flow exceeds a certain threshold level," said Temple mathematician Benjamin Seibold, "so the obvious way to prevent phantom jams is to reduce the flow."