More than a dozen semi-autonomous trucks on Wednesday completed the world's first cross-border truck platooning experiment.

Truck platooning involves linking two or three trucks in a convoy; each vehicle closely follows another at a set distance, using connectivity technology and automated support systems. Like a traveling conga line, the rig at the head of the platoon acts as the leader, with the rest reacting and adapting to changes in its movement. So if the leader brakes, all the other trucks brake, too.

In this experiment, six groups of self-driving trucks from DAF, Daimler, Iveco, MAN, Scania, and Volvo left factories in Sweden, Germany, and Belgium, following routes longer than 1,200 miles to the Netherlands. The squads arrived in Rotterdam's harbor yesterday in "truck platoons."

The teams platooned only on highways in normal traffic conditions; if weather was bad or gridlock too heavy, the trucks would temporarily uncouple, then sync again later. Human driver were on board in case of an emergency, The Guardian says.

"The only successful European Truck Platooning Challenge will be a safe one," the EU agency in charge of the program said.

What may sound like a show of strength or simply an advertisement by truck makers is actually a high-stakes experiment. This challenge was meant to highlight platooning and push it to the top of the EU policy makers' agenda. Supporters say the process will make transportation safer, cleaner, and more efficient, and reduce fuel consumption through improved driving practices. Plus, it has the potential to reduce CO2 emissions by up to 10 percent, according to Iveco parent company CHN Industrial.

The news comes as Volvo announced plans to test self-driving cars in Chinese cities in the coming months.

Here in the states, meanwhile, Google is expanding its self-driving car trials into Phoenix, Ariz. The fourth U.S. city to serve as a real-world research center for the Web giant's autonomous technology, the Valley of the Sun lends a decidedly dry setting to vehicle experimentation.

"This new location will give our cars a chance to practice driving in a desert environment—and experience handling some of Phoenix's quirks, like cacti, golf cart crossings, and time-based speed limits," the Google Self-Driving Car Project wrote in a Google+ post.