I think you underestimate the difficulty of controlling a junction of that size with human beings. It's a 3-lane roundabout with 12 lanes of traffic feeding into it. There are effectively 4 separate junctions where traffic conflicts need to be controlled.
The basic problem is that, without the flyover, the junction does not have the capacity to deal with the total volume of traffic on A4 and A316, particularly since most of the northbound A316 traffic wants to turn right on to the A4 eastbound, in conflict with A316 southbound traffic from Dorchester Grove.
I think the best that could be done would be to tweak the traffic light phasing a little, to give A316 northbound traffic a longer phase, but you'd then get more jams on the A4 and on Chiswick Lane southbound. There is no easy answer in my view.
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