You'd have though by now that National Highways would know that. Oh wait, that assumes NH is a logical organisation which learns from best practice etc...
A prime example of this is the M25/A3 junction. Currently a 3-level roundabout with traffic lights, the preferred upgrade is... a slightly bigger 3-level roundabout with traffic lights. What an imaginative idea! It'll definitely solve all the problems...
This is an interesting point. Round here (Hertfordshire) there’s been the ongoing saga of the Welwyn-Stevenage section of the A1M, which is, except for some tinkering with markings, unchanged since the 1980s despite carrying more and more vehicles. Covid has given it a bit of respite, but before that it was essentially a black spot, when barely a week passing without at least one serious accident, and gridlock to the whole surrounding area in the event of the slightest problem.
In this instance I suspect widening wouldn’t totally remove the issues, as what would happen is a lot of traffic which currently avoids the motorway by way of other routes, including country lanes, would simply revert to the motorway.
There might be a slight improvement where for example at the moment there are regular holdups simply because a 2-Lane motorway can’t cope with the volume of joining traffic, but without some works to surrounding roads layouts these would simply become the limiting factor. No point enhancing the capacity of a motorway if the surrounding roundabouts can’t take the traffic.
Having said all that, they should just get in with widening this section, one way or other it needs it. It should be a proper widening too, not just a smart conversion, but I bet the latter is what will happen.