The N2 between Grabouw and Blikkiesdorp in the Western Cape has been closed in both directions because of a violent protest.
Western Cape traffic chief Kenny Africa told News24 that the road was closed as a preventative measure to protect motorists from protesters throwing "rocks".
Africa could not say what the reason for the protest was.
"People are throwing rocks at cars," he said, "and until police have contained the situation, the road will remain closed."
He said there was a heavy police presence in the area and that there had been no clashes between police and protesters.
In April, the N2 was closed for two days amid service delivery protests by residents in the area.
Africa advised motorists to use alternative routes.
"From Caledon, turn left at Botriver towards Kleinmond via the coastal route to get back on the N2 at Gordon's Bay. When travelling from Gordon's Bay, take the coastal route through Kleinmond to get back on the N2 at Botriver."