From 15 years of living in Bristol and North Somerset, you really can't beat A/M27, M3, A34, M4 (then either M32 or M5/Portway) if you get a decent run. You may only have to drop below 70 (80, 90, whatever) only twice between Chichester and Bristol.
...and picking up your later post - there's a...
Loads of on-street parking, mainly to the immediate South and East of the stadium as it's all residential. Aim to park a little bit of a walk away so you don't get snarled up getting away after.