He told me that after thinking long and hard about the problem caused by the one-way street, he came to the conclusion that the only solution was to have a lop-sided roundabout. So he drew it up and showed it to his team leader. The team leader saw how unusual it looked and made to explain how it should be improved, but then stopped because he couldn't actually think of a better way of doing it. So the team leader passed it on to his boss Brian Raistrick.
When assessing the merits of a highway design, Raistrick would hold the plan horizontally, level with his eyes, and mentally drive through all the routes, turning the paper as he went. Again, Raistrick found my dad's design unusual, but couldn't find anything wrong with it, and so it was built.