Self-driving cars have been heralded as the answer to problems from tedious roads and congestion to traffic accidents – more than 90 per cent of which are caused by human error. Numerous companies and cities have conducted trials. Google has clocked up almost 1m miles of testing on public roads in the US and four cities in the UK will host pilot projects early in 2015. But technical and legal experts are increasingly concerned that the frenzied excitement surrounding the technology is obscuring the significant obstacles that lie in the way of the introduction of autonomous vehicles.