A number of "homemade devices" have been found along the O-Bahn track, north-east of Adelaide.
A section of the O-Bahn was closed as police investigated and buses were forced to take an alternate route.
SA Police said officers found a number of suspicious homemade devices at the Campbelltown section of the busway.
Emergency services were called just before 9:00am after a member of the public found one of the devices.
The bomb response unit was called to the scene and found "homemade incendiary devices".
"The devices were not explosive," a statement said.
Police have urged anyone with information to contact CrimeStoppers.
Earlier, the State Government revealed permanent anti-throw screens had been installed on all bridges along the Southern Expressway.
It followed a spate of rocks being thrown from bridges onto traffic.
Transport Minister Stephan Knoll said existing bridge CCTV had also been upgraded.
He said the Government would spend a total of $15 million on the upgrade.
The project is expected to be completed by June.
