As suggested, it could be that the traffic lights on the Hogarth roundabout give precedence to 'rush hour' traffic leaving town until around 19.30, when it gives more time to 'ground level' A316 traffic.There's also the intentional ban on through traffic sullying the avenues of Grove Park and having to use the roundabout, which doesn't apply after 19.00.
Peter Evans ● 4d