Self-driving cars at the moment are designed on the "all else being equal" principle - the roads, the signs, the jaywalking pedestrians and the other drivers all remaining a constant factor. But what if the road was imbued with signage: stop here; turn there? Make a lot more sense, surely? In the week-old copy of The Economist I was reading earlier, there was a piece on training algorithms to recognise stop signs - clean stop signs, dirty, damaged and graffiti-market stop signs. Really?