The A420 has been closed in one direction outside Gablecross Police Station after a trailer lost its load.
Police were called to the scene at around 6.45pm after the tractor spoilt dozens of hay bales across the Oxford-bound carriageway.
The driver was uninjured in the incident and no other vehicles were involved.
The A420 between Gablecross roundabout and Gablecross Police Station is expected to be in place for around two hours.
Police say they will be investigating the cause of the crash and dealing with any motoring offences found.
