Imagine a train: A bunch of cars linked together being pulled by the front car. Take that same basic concept, but replace the cars on tracks with cars on the road, and the physical connections between them with wireless connections, and you got a road train. Safe Road Trains for the Environment (SARTRE) Project is currently working on developing these systems in an effort to improve the environment and road safety.
This is how it works: A professional driver controls the road train in the lead truck to form a convoy of cars that are semi-autonomously controlled by the front truck. The front truck communicates things such as speed and directional controls to the other cars, enabling them to follow its path exactly. The cars themselves also have sensors measuring the distance to the other cars around it, creating a fallback safety mechanism in case something happens to the truck.
The benefit of this system would be reduced average speeds--and therefore reduced fuel consumption and traffic congestion. Volvo, one of SARTE's partners, demonstrated that car train worked with a single car joining the train, becoming "enslaved" by the lead truck, and than successfully exiting. They plan to do further tests with more cars and in street traffic soon.