Looks like the old Dorchester Hill coming out of Blandford on the old A354?
It’s such a shame that although you can get through the hedge onto the new A354 there is no footpath or bridleway opposite so unless you fancy walking along an A road only option is turn back.
Up on the old Dorchester Road, Go over the bridge to Blandford St Mary, keep going straight past Stour Inn until you come to a large gate, where you can walk through and up the hill.
The old road into town before the bypass was built, connects onto the A354 to Dorchester, it went under a bridge pretty much where the round about/tesco petrol station is. Apparently it’s still there buried according to local folklore.
Extract from Abanoned Roads group on Facebook