GJD wrote:Have I pictured this right? You were on the westbound M4 (albeit locally there it's more northbound than westbound), you exited at junction 28 and at the roundabout you wanted to take the westbound A48 - it that it?
Doing that you would pass through two sets of traffic lights on the roundabout - you're talking about being stopped at the first set?
When the driver behind the van beeped you, were you changing lanes into the space in front of them?
There's can certainly be a case for preferring to conform to the prevailing traffic pattern than the instructions of some white paint.
I was on M4 eastbound (i.e. coming away from Cardiff) and at the roundabout wanted to go towards the third exit towards Newport, which I think is the A48 eastbound. I was in the middle lane at the lights before the exit that would go back on the M4 eastbound, the lane was marked as being for the M4 or A48 Newport. The lights are shown here on streetview: http://goo.gl/maps/A6eba.
When I was beeped I wasn't changing lane, I was heading straight in order to turn smoothly into the first lane to the right - the technique suggested by Gareth above.