I don't want to interfere, but I suggest that there is another factor in all of this.

The exit from Devonshire Road onto the Sth Circ can be seen clearly by traffic in both directions, whereas traffic coming round from London Road is virtually on top of the exit of Davids Road when it comes into view - not to mention the fact that pulling out from Davids Road is like a game of chicken!

Also, if the traffic's backed-up at the lights and a vehicle wanting to turn right from Davids Road pulls halfway across, that could potentially cause a log-jam by blocking the traffic on the Dartmouth Road phase which, in turn, could back-up across all 4 lanes of the main junction.

By the way, I remember Davids Road when it was 2-way (and it was great for the old horse 'n' cart!).