Stop-and-go waves of dense freeway traffic can flow backward for miles. This TED-Ed video explains the phenomenon of "phantom traffic jams" and shows how self-driving cars that are aware of the traffic conditions ahead can adjust their behavior to prevent phantom traffic jams. Interestingly, models have shown had just one self-driving vehicle for every 20 human-driven vehicles can dampen a stop-and-go phantom traffic jam wave.
Image: YouTube
