M4 re-opens after overturned lorry shed concrete onto road
The lorry was travelling eastbound before careering into the westbound carriageway about one mile from Junction 14, spilling its load.
A passenger was airlifted to hospital, but his injuries were not thought to be life threatening.
The driver was also taken to hospital to be checked over.
Thames Valley Police spokesman Adam Fisher said no other vehicles were involved.
A diversion was put place and delays were considerable.
Lanes were closed to help clear the scene, and as of 2.30pm were all re-opened, however traffic was still heavy.
Drivers have taken to Facebook and Twitter to express their anger at being stuck for more than 90 minutes in tailbacks.