Nissan has announced its Dayz Wider Kei car will be equipped with semi-autonomous assistance with acceleration, steering and braking on the highway.
If such Kei Cars are used, then the width of a standard Australian traffic lane (3.5 m) could carry two lanes of oncoming traffic. Conventional vehicles, such as fire appliances, could use this road, with the oncoming traffic diverted to a passing loop (as is done with railways).
This would greatly reduce the cost of building a highway, as it would only need to be 3.5 m wide for most of its length. Where cost and space are at a premium, and conventional vehicle access is not needed, an even narrower single lane roadway could be used. This would allow bridges and elevated roadways to be quickly built from shipping container sized modules. Tunnels would only need to be about 3 m in diameter.