(18 Apr 2015, 12:56 pm)Andreos1 wrote Probably to move traffic away from the city centre/shopping area.

Up until the mid/late 90's, the very top (onto Northumberland Road) was still open for buses.
The remaining section still had restricted access and a small road going down the middle for a long time. Possibly into the early 90's.

Edit: http://www.skyscrapercity.com/showthread.php?t=982536&page=113 found this, with images of the changes and the small road that wound its way down.

The top bit of Northumberland was pedestrianised in 1999. The bottom bit, I'm guessing, 1989/90, perhaps earlier. I believe the street was narrowed once the Central Motorway East was completed in 1975.

The picture below (perhaps my favourite) was taken in 1976.