I am a bit surprised at some of the suggestions – in a Bristol rush hour (only its longer than an hour both ends of the day) the traffic is very heavy both entering/leaving it and crossing the city.
There is no way at all some of the suggested areas will let you access Filton in 30 mins. Many wont make it for at least an hour or quite a bit longer – the road route to Bath is a good 40 mins when there are no queues at all never mind in rush hour. Travelling across Bristol is very dependent on which area you start from. It can be very trying.
To grasp the commuter problem and how bad it is around the Filton and South Glos areas, look on a map. It can take literally take over an hour to travel from say just Winterbourne to the MOD at Abbey Wood area in term time for example, which is a 15 min drive tops the rest of the time. A friend of mine regularly finds this. I have travelled between the hotel by the UWE Frenchay campus to Parkway train station in mid winter rush hour peak and its taken nearly an hour! It possible to walk that distance in less than 40 mins.
Try looking up the Western Daily Press, Bristol Evening Post or Thornbury Gazette for stories on congestion and people complaining that they are still building houses despite the lack of infrastructure.
The costs of traveling over the Severn Bridges are already over £5 per day, plus fuel etc and the price goes up every January. Often the older bridge is shut due to weather problems which causes big diversions.
Its mostly a nice area when out of rush hour but I really would suggest you not be taken in by how close things look on a map and assuming the commuter time is therefore equally short.