"we are told that [Hammersmith Bridge's] closure has not increased congestion on Putney Bridge because all the drivers who used to cross at Hammersmith have simply evaporated, using public transport instead. Is that plausible?"Of course it isn't plausible, because you are wrong. I don't know how your fevered imagination conjured up this fantasy, but in the real world the figure is not "all the drivers" but around 40%, the other 60% being distributed over other bridges, mainly Chiswick.
Richard Carter ● 6d