The northbound A41 was closed after an accident caused a car to flip onto its roof early yesterday afternoon.
Emergency services were called to the Bourne End junction of the dual carriageway at about 12.50pm.
The driver of an overturned car had been injured after a collision between a red Mitsubishi Colt and a silver Ford Focus.
Police closed the carriageway just before 1pm while paramedics treated the motorist and it was reopened just after 2.20pm.
Ambulance service spokesman Richard Dunne said: “We were called just before 1pm to a two vehicle collision treating a man in his 50s who suffered minor injuries.
“He was taken to Hemel Hempstead General for further treatment.”