We started off with a really bad model - build narrow and then widen, so that slip roads, bridges etc need to be re-built at any time we want to add a lane. A far better model would have been to build wide apart with the space between the carriageways separated with enough space to add a couple of lanes each way. In that way, the hard shoulder, on and off ramps/slipways, bridges etc would not have to be re-built. It is this precisely the cost of all these things that led to the promotion of "smart" motorways as a way of avoiding upgrade costs. Short term thinking = long term costs.
Christopher Gillie ● 2022d