The pedant in me wants to point out that Bristol, like most old world cities, is not capable of being gridlocked as its streets do not form a grid - but that would be a digression, so I won't.
If Dr Steve Melia is to be believed ( see 'Urban Transport Without The Hot Air',
http://www.stevemelia.co.uk/urbantransport.html ), even a high-quality tram-style transport system would probably have little impact on congestion - and of course MetroBus is anything but a tram system; rather, it is a trojan horse for road-building.
Ironically, though, it is largely the lane restrictions that have been put in place while The Centre is remodelled for MetroBus that are making congestion worse than normal.