I think it would be best if you could record a short clip in game showing the behaviour :)
Also check lane connections, maybe they are forced to do so (there is no other option or they are algorighmically worse when applying pathfinding penalties)

Not to worry, i turned on dynamic lanes and it seems to solve the issue :)

Yes, that feature helps smoothing the traffic by altering original path based on current road conditions. Try to not put it to the highest value because vehicles will change lanes more aggresively usually ignoring the fact it may reduce traffic flow. Lower values value more the lanes that can increase road traffic flow over the vehicle speed improvement if the outcome diffrence is relatively small between options.

when I have some time I will experiment, but, I think it is the aggressive switching gives a more realistic impression of the way cars drive in the real world.

I'm not so worried about flow as I'm not using the mod to fix traffic problems per se, My solution to traffic is to be cleverer with zoning and have multiple routes.

I use the mod to try to prevent the cars tailing each other in the left hand lane using lane arrows at the junctions.

and now that I have turned on advanced AI and dynamic lanes, it seems to be working.

On a slightly different issue, if whne CTRL CLICK a junction and one of them is a four lane two way, the lane arrows are exactly incorrect most of the time and have to be CTRL CLCKD again to put them right.

That is to say instead of having the left hand lane as the turn only, it chooses the right hand lane as the turn only.

I assume this is to simulate right hand slip lanes at lights from places where a right turn can ignore the red. But where I live, it is the turning that crosses the traffic which has the dedicated lane as that is the one that gets held up at the lights.

A small point and it is trivial to correct, but I wonder if it is random or weather there is a logic to it?