The A23 in Bolney was closed so that police could help a person on a bridge, a police spokesman said.
Police were called to the A23 near Bolney Cross and the junction with Cowfold Road at 5.05pm following reports of a person in distress on a bridge there.
A police spokesman said they were concerned for the person's welfare, so officers closed the A23 at 5.25pm to engage with them.
The person was helped off the bridge and has now been 'detained', a police spokesman said.
The road was reopened at around 5.50pm, police said.