The left hand lane heading towards Gosnells is ALWAYS packed because people hug the lane to turn left into the shops all along that area from manning road to welshpool road. The traffic lights dont help too. Finally its a busy street with a lot of busses so when a bus stops thats it. Grid lock.
I always drive the right lane, then just before carousel cut into the centre. Then just after the lights after the servo and first turn off to get to carousel go into the left lane. Then at the next set of lights get back into the middle. Stay in the middle all the way because from here the right lane is plugged up with people trying to get to nicholson. If you do need to turn down there wait until the last second as the delay is so long theres always a break in the que to get in. The other option that works well is simply to drive straight and do a U-turn opposite JB hifi. You will 'beat' hundreds of cars.
Its just a very poorly designed area. The three lane road is trying to service all of south perth. The public transport is bad (no parking at any station plus you get broken into all the time) so many people drive.
It depends on the time of day, and which day you are travelling past Carousel - if heading toward Gosnells in the left hand lane, especially during peak periods and saturday morning/lunch times it can get quite slow. There is a turning lane out the front of where Harvey Norman is into the parking area - this can get backed up if the car park is full so that is generally the reason.
There is also a bus station just before the turn in lane, so when busses pull out, this can also slow down traffic :)
If I am heading toward Beckenham/Gosnells on a Saturday I usually go up Cecil Ave through to Sevenoaks St and take that to William/Albany Hwy to avoid the major back ups :)