As a former resident of Wrington (just south of the airport) who commuted into Bristol, it is my contention that the congestion Bristol suffers is caused by the river/docks in the city centre, and the lack of bridges. I now live north of Bristol, and when travelling to the airport will always try to approach it from the south, usually by way of the M5 and A370 or via Yatton. Further, but quicker.
I hope that Bristol will get its 10 million pax cap removed as it is a popular local alternative to the crowded jungles of Heathrow and Gatwick.
The idea of a planning "cap" seems a rather Luddite approach to the increasing demand for air transport, imposed by backward-looking councils to emphasise their own importance.