My GF lives in Chepstow, I'm Peterborough. I used to go M5 all the way in the early days and then I learnt about the M50. It's quite a few miles shorter and even if you get stuck behind something slow on the last stretch it works out almost the same and avoids bridge tolls!
It's also a really pretty drive and tintern at night is lovely too. You don't need speed bumps to make people go slow, just really pretty things worth taking the time to look at
