We know that Boris Johnson’s fantasy of “smoothing traffic flow” will act as an incentive for people to get into their cars and, even more so, for businesses to move more stuff around. In a city like London there is much more potential demand for road space than could ever be supplied, because individuals and businesses who see an empty bit of road will always conjure some reason to fill it. An equilibrium is maintained by the tolerance that individuals have for sitting in traffic and the tolerance that businesses have for spending money doing business on the roads. Add or remove capacity to London’s road network and it will not make the slightest difference to congestion or journey times. It’s not like we haven’t tried it enough time to be sure of that.