The legislation roadblock
Cars are obviously capable of finding their own way around already, so what keeps the autonomous automobile from completely taking over our streets? Simple: The 19 Convention on Road Traffic, drafted in Vienna in 1968 (PDF), which doubles as a UN treaty. Article 8 of the treaty contains the vital point: "Every driver shall at all times be able to control his vehicle or to guide his animals". Countries like Germany or Austria ratified the agreement and incorporated it into their local laws. Other countries, such as the UK or Japan, signed the treaty but didn't ratify it completely. The US didn't sign the treaty at all, however - making it possible to drive cars autonomously in California and Nevada.