The first driverless cars carrying passengers are already on the streets. Some experts say this is just a beginning. Before conquering the roads however, robot cars still have a long journey ahead.

As the vehicle glides away from the kerb, only the whine of the power-steering pump can be distantly heard.

This Mercedes-Benz van has just a joystick and an emergency override button on the fascia - there is no steering wheel.

In the test version, the button would allow an attendant to take rapid action if needed, but for all intents and purposes, this test van from German automotive component maker ZF is doing all the driving here - without any human intervention.

"We want to push ahead with this next mobility step," said ZF chief Wolf-Henning Scheider at the firm's base in Friedrichshafen.

Engineers call this degree of full automation Level 5, which means no human interaction is required. The car is able to go anywhere and do anything that an experienced driver could do.

At the moment, cars in this category are still experimental, and they cannot be seen performing on public roads anywhere around the world.

A notch down are Level 4 autonomous cars, which are equally high-tech but allow the driver to intervene if anything goes awry. Typical of these are minibus shuttles with a human conductor on board as a precaution.

Companies close to the technology say the world of robot cars is only a few years away, but experts say ensuring the safety aspect is vital if people are to feel comfortable travelling in this way.

"To achieve this stage, autonomous driving must be 100% safer than driving with a person in charge," said Schneider. "Only then will they be accepted by the public."

It's an unusual feeling sitting inside a vehicle that moves around without any human hand to guide it. The familiar steering wheel is missing from the cockpit, and setting off is a jerky affair. Most learner drivers would manage to pull away more smoothly.

The robot van brakes and steers around obstacles that suddenly appear with aplomb, but its changes of direction are abrupt.

After a few circuits of the test track, passengers loosen up and feel less nervous. They start consulting their smartphones instead of staring at the road ahead, poised for evasive action. It is soothing to know that the test track is fenced-off from regular roads and the top speed of the van is governed down to a maximum of 20 km/h.

There are faster autonomous cars around, although they are hardly common on roads. The Cadillac CT6 with its integrated "Super Cruise" can travel at up to 137 km/h in a robot mode. To do this, it must keep to North American highways, which have the sensors to support its sophisticated guidance systems.

The on-board computer coordinates data from a battery of cameras, sensors and laser grid-map navigation to keep the car on a pre-set path. Drivers just activate the Super Cruise button before taking their feet off the pedals and letting go of the steering wheel.

Anxiety remains however, and most owners of this clever luxury car will find their hands hovering over the wheel during a journey, just in case they need to take over in a hazardous situation.

Cadillac currently offers the Level 3 autonomous system only in the United States and Canada, and so far, CT6s with the gadget have covered 215,000 km in the two countries.

Level 3 vehicles still require human override, but they can detect the environment. This means they can make informed decisions for themselves, such as whether to accelerate past a slow-moving vehicle.

The flagship Audi A8 has Level 3 capability, but the technology has yet to be rolled out on German roads, where laws still will not permit it. The next-generation Mercedes-Benz S-Class is also rumoured to be similarly equipped.

The consulting company Berylls Strategy Advisors has simulated the use of Level 5 robot taxis in the German city of Munich. It concluded that 18,000 of the driverless buggies could replace 200,000 existing cars and carry 20% more passengers in the bargain.

The figures are impressive, but Berylls CEO Jan Burgard warns against high hopes in the near future. "The results are not likely to be realisable within the next 10 to 15 years owing to the existing traffic situation in most large European cities."

He sees Level 4 and 5 cars gaining traction in some cities sooner, but only if laws, weather condition and traffic density permit.

A lot of administrative and legal hurdles will need to be cleared in Europe before robot cars take to the streets in significant numbers. Progress in less regulated countries like China, the Middle East and the United States.

German automotive expert Stefan Bratzel is even more cautious. He believes only a single-digit percentage rate of robot cars will be achievable by 2030.

So are robot cars still a pipe dream? Some progress has been made, such as in the small German resort town of Monheim on the River Rhine where passengers have been sampling a driver-less shuttle since autumn 2019.

The little buses ferry groups from the railway station to the town centre every 10 minutes. The minibuses operate on public roads at speeds of up to 20 km/h. A guard is always present on board and can take action if danger looms.

In Bad Birnbach in Bavaria, a similar shuttle travels over a 1.4-km stretch at up to 15 km/h. A maximum of six persons can be carried.

ZF has meanwhile teamed up with Aachen-based start-up e.Go Mobile to offer its own boxy minivan called "Mover." Not just a concept, the driverless bus can carry up to 10 people and is fully electric. It entered production this year and is set to operate at locations in France and Germany.

 - dpa