While bypass is the best and probably cheapest solution, it has been ruled out because it is in the Green Belt.
However, houses are being allowed to be built on it. This is nearly as daft as allowing houses to be built on flood plains. The committees involved are obviously unfit for purpose.
The only other practical option is a flyover.
I suggested a flyover scheme from Offington Corner (pictured above) to Lyons Farm in January.
It would come down to the lower level on the dual carriageway just after Lyons Farm. This would allow through traffic to go onto the local roads and vice versa.
It could then rise again at the Hillbarn junction as suggested by Lionel. His suggestion that sections should be done as money becomes available is excellent. If the Monks Farm development goes ahead further alterations would be needed.