Having worked on both the claimant side and now the defendant side for RTA claims, the classic case law always quoted is Wadsworth v Gillespie (1978) which is 2/3rds / 1/3rd split liability with the party proceeding along the main road getting 2/3rds (lose 1/3rd for misleading signal).
But this is an old-ish bit of case law and you may hear Soils Ltd v Bromwich (1998) or Davis v Swinwood (2002) quoted now.
Soils Ltd was 100% to the driver on the main road, as they did indicate left but gave evidence to confirm that they were pulling up to park on the main road just after junction.
Davis v Swinwood again 100% in person on main roads favour.
Basically the Courts these days seem to take the view that even though signal is mis-leading it should not be relied upon.
So if you were the one on the main road and they start offering 2/3 to 1/3 quote the more modern cases above. If you’re the pulling out, try and fight for one third in your favour.