Imagine motoring at high speed along the autobahn, just inches from the vehicle in front. Behind you, another car is following equally closely. Look across to your driver and he or she is reading a book. Welcome to the future – a possible future if autonomous vehicles take to the roads.

Though trials of autonomous cars on public roads are now legal in parts of the USA, and the necessary technologies are becoming increasingly available, there are still some serious challenges to be overcome before fully autonomous vehicles actually take to the roads.

There are three types of issue to be overcome: technical, legal and marketing.

The technical challenges relate to communication of signals (interference, security, recognition), system reliability (both hardware and software) and compatibility issues between autonomous and manual vehicles in proximity. Imagine the challenges posed designing a system capable of reacting appropriately when someone in a manually piloted car interrupts a group of autonomous cars travelling in convoy,  or the difficulties of devising a test program so exhaustive that it can recognise and differentiate between the myriad objects moving around in a city centre an react accordingly.

Things get really complicated when you have an accident. Was it your fault or did the system fail to respond correctly? Does the other driver sue you or your car’s manufacturer? If, as seems almost certain, partially autonomous vehicles become a stepping stone to full autonomy, you can never relax behind the wheel because you may have to intervene manually at any time.

And if the systems in a semi- or fully-autonomous vehicle keep you safer by avoiding accidents, how does the vehicle manufacturer defend making manually controlled vehicles at the same time, that are, logically, less safe?

The marketing challenge is to convince drivers that they actually want it. Surveys around the world suggest a slight majority in favour, presumably attracted to the lure of more relaxing leisure journeys and more productive business trips. Interestingly, many of those same people are against the idea when asked if they’d put their children in the car. What does that tell us? And how will people react to any accidents caused by an autonomous vehicle?

Cost could be another obstacle. It will require aerospace levels of reliability in the software, to ensure the necessary safety standards, and that will add considerably to the system costs. Presumably, as with any new technology, early adopters will pay a high price but costs will fall as manufacturing volumes increase.

While the end goal of widespread use of autonomous vehicles may have safety, convenience and fuel economy benefits, it seems that the biggest hurdles will arise during the transition when the vehicle population is mixed. The combination of manual, semi- and fully-autonomous vehicles in the same road space will dilute the benefits of the fully autonomous technology and complicate the engineering required to introduce it.