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Chris Urmson, director of the Google car team, said this is because the detection technology is not yet strong enough to separate certain objects from weather conditions.
While the cars' cameras can spot a traffic light changing, they can be confused by strong sunlight.
They don't distinguish between an empty plastic bag - which could be easily driven over - or a rock, so cars must drive around both. They also can't detect uncovered manholes or potholes.
Mr Urmson told the publication: 'I could construct a construction zone that could befuddle the car.'
The cars 'see' pedestrians as moving blocks of pixels and know to stop, but unlike a cautious human driver, they could not spot a traffic policeman at the side of the road, waving for traffic to stop - which could lead to trouble.
Despite the cars being allowed on public roads, they need to prepare to set off in more detail than a human driver, because a precise map must be created of exactly where to go - and a car cannot deviate from its route.
An area has to be mapped multiple times by a sensor vehicle to record details such as driveways, in order to make the cars' routes.
These details then need to be pored over metre-by metre by humans and computers, in a much more labour-intensive process than in needed to maker Google Maps.
Despite best efforts, routes can lack details such as temporary traffic lights that have been installed, or if there are road works or potholes to avoid, which the car might drive over.
In addition, maps have only been made for a few thousand miles of road, so in order for the cars to roam the US, Google would need to pour a vast amount of resources into mapping the rest of the vast country, down to the tiniest changing detail.
The company is working on ironing out all these problems. For example, if a Google car notes new lights and street signs, it sends a message to update the mapping software.
Google unveiled a compact version of its car without a steering wheel, but it will now have to re-introduce the wheel to comply with the California Department of Motor Vehicles' new regulations.
Despite all these set-backs, Mr Urmson said autonomous cars will 'happen more quickly than people think' - perhaps as soon as in five years' time.
HOW DOES GOOGLE'S AUTONOMOUS CAR WORK?
The prototype two-seater cars have buttons to autonomously begin and end the drive.
The car makes turns and react to other vehicles and pedestrians based on computer programs that predict what others might do, and data from sensors including radar and cameras that read in real time what other objects are actually doing.
The route might be set by typing a destination into a map or using spoken commands according to Chris Urmson, the leader of Google's self-driving car team.
The car will be powered by electricity and could go about 100 miles (160 km) before charging.
Its shape suggests a rounded-out Volkswagen Beetle - something that might move people around a corporate campus or congested downtown - with headlights and sensors arrayed to resemble a friendly face.
The front of the vehicle has a soft foam-like material where a traditional bumper would be and a more flexible windscreen, in a bid to be safer for pedestrians.
In these prototypes speed is restricted to 25mph (40 km/h) and the ability to self-drive will depend on specifically designed Google road maps tested on the company's current fleet of vehicles.
Ultimately the vehicles will be faster and will be able to use Google's extended maps service. Driving works by using GPS technology to locate the vehicle's exact position on an electronic map.
A combination of radar, lasers and cameras sitting on top of the roof give the car a 360-degree 'view', with sensors linked to computer software able to 'see' and identify people, cars, road signs and markings and traffic lights.
-Daily Mail
