- Volvos equipped with cameras, radar and laser sensors, enabling them to monitor the lead vehicle.
- The lead vehicle drove the road train 124 miles along a motorway outside of Barcelona.
A convoy of self-driving cars has taken to a public motorway in Spain in normal traffic, a world first, according to Swedish car maker Volvo.
A professional driver took the lead of the convoy in a truck, and was followed by four self-driven Volvo vehicles: a second truck and three cars, Volvo said in a statement.
Vehicles in the road train were equipped with safety systems including cameras, radar and laser sensors, enabling them to monitor the lead vehicle and other vehicles on the road, Volvo said.
"By adding in wireless communication, the vehicles in the platoon mimic the lead vehicle using Ricardo autonomous control - accelerating, braking and turning in exactly the same way as the leader," it said.
The cars successfully drove for 200 kilometers (124 miles) on May 22 along a motorway outside Spain's northeastern city of Barcelona, a Volvo spokesman said.
Volvo Car Corporation's project manager, Linda Wahlstrom, was filmed driving one of the cars in the convoy as the system instructed her to lift her feet from the pedals and then remove her hands from the wheel.
As the car sped along the highway at 85 kph (53 mph), she leafed through a magazine.
"It is quite funny to see the passing vehicles. They are quite surprised seeing me not driving the car but reading a magazine," Wahlstrom said.
"We've learned a whole lot during this period. People think that autonomous driving is science fiction, but the fact is that the technology is already here," she added in a statement.
"From the purely conceptual viewpoint, it works fine and the road train will be around in one form or another in the future."
It was the first-ever test drive of a self-driving road train among other road users, Volvo said, describing the trial as "highly successful".
"The project aims to deliver improved comfort for drivers, who can now spend their time doing other things while driving. They can work on their laptops, read a book or sit back and enjoy a relaxed lunch," Volvo said.
"Naturally the project also aims to improve traffic safety, reduce environmental impact and, thanks to smooth speed control, cut the risk of traffic tailbacks."
The close distance between the cars also creates a slipstream that allows the vehicles to use less fuel, it says, with savings of up 20 percent possible depending on spacing and geometry.
The Safe Road Trains for the Environment (SARTRE) project is a partly European Commission-funded joint venture led by British engineering and technology developer Ricardo UK.
Other firms collaborating in the venture are Volvo, Idiada and Tecnalia Research & Innovation of Spain, Institut für Kraftfahrzeuge Aachen of Germany and SP Technical Research Institute of Sweden.