Motoring organisation the RAA said designated bus lanes on Anzac Highway in Adelaide had slowed traffic flow.

It also said commuters were taking longer to drive between Adelaide's southern suburbs and the city on South Road.

The average speed on parts of the drive were put at 17 kilometres per hour in the morning peak and 25 kilometres per hour in the afternoons.

Mark Borlace of the RAA said it wanted the Government to review using designated bus lanes.

"They should re-think what they've done here because people still aren't using public transport, for all the other reasons, frequency, cost, you know all that sort of stuff, or [the Government needs to be] having intelligent systems that at least tell people how far away the real bus is," he said.

Transport Services Minister Chloe Fox said she had received positive feedback from passengers about the use of bus lanes.

She attributed slower driving along South Road to the rail line upgrading work and the duplication project for the Southern Expressway.

"The Noarlunga [rail] line is shut, there's extraordinary work going on duplicating the highway. What that means is a huge amount of people who wouldn't normally use it are using South Road, arriving at the Gallipoli Underpass intersection and turning right (into Anzac Highway toward the city)," she said.

Ms Fox said travel time for bus passengers had improved significantly, but patronage had only increased on some routes.

"I need to be honest here and talk about why that might be. That also might be, beside the time which has been shaved off the bus journey, obviously there are certain lines there, the 265, the 263, they're customers who have been directly affected by the Noarlunga closure as well, so there are different reasons there," she said.

The RAA said it wanted the SA Government to release its South Road planning study on the overall strategy for upgrades of Adelaide's main north-south transport corridor.

In response, the Government released a 29-page document hours later.