take your map and look at Milton Park, south mead estate on the other side of the power station site, and Harwell a mile or so in the other direction....and bear in mind the three sites have around 10k jobs. Then plot routes from Grove, Wantage, the Hanneys/denchworth/southmoor, and many other local villages and suddenly the concerns make a lot more sense. The other routes are all at capacity to the point a single broken down car on any one of them can cause 30 minutes or more delays to many many people.
Also bear in mind that the other main route out of Steventon, (especially if after A34) being three miles through Drayton to Abingdon can take 30 minutes or more in rush hour on a normal day.