A deepening rift over a plan to lower speed limits on some of South Australia's regional roads has taken another turn with the State Opposition saying it would reverse the government's decision if it was elected.

At the same time, Labor Government minister Geoff Brock has broken ranks and is calling on MPs whose electorates will be hit by the lower limits to lobby against the move.

Last month the Department of Transport and Infrastructure announced it would lower speed limits from 110kph to 100kph by the end of the year on eight regional roads and highways.

Lower speed roads Browns Well Highway

Browns Well Highway Riddoch Highway

Riddoch Highway Clay Wells Road

Clay Wells Road Southern Ports Highway

Southern Ports Highway Andamooka Road

Andamooka Road Cleve Road

Cleve Road Ngarkat Highway

Ngarkat Highway Goyder Highway

Goyder Highway Carpenter Rocks Road

Opposition spokesman Stephan Knoll said that decision would be reversed if his party won the next election next year.

"We think the government has waved the white flag in terms of getting road infrastructure up to scratch, and is instead taking the easy option of continuing to lower speed limits," Mr Knoll said.

Constituents' concerns

"We think enough is enough and with these eight roads is where we draw a line in the sand.''

Mr Brock, who is Minister for Local Government and Regional Development, said he wanted to make it clear that he disagreed with his government on the speed limits.

However, Mr Brock said he was not speaking as a cabinet minister, but as the local Independent Member for Frome.

"I have had my discussions and I will continue to voice my disapproval for this move across all regional South Australia," Mr Brock said.

Geoff Brock: Lobbying against his government's speed limit plan. ( ABC News )

Mr Brock said his constituents were concerned and he encouraged other regional politicians to lobby the State Government.

"What other elected members representing the rest of the regional areas out there should be doing is what I've done.''

Mr Knoll said the Opposition Liberal Party's policy was a balance between road safety and the need for regional South Australians to get around efficiently.

"There are often multiple causes to any car crash,'' Mr Knoll said.

''It can be from poor roads, excessive speeding, drug and alcohol abuse, driver inattention and fatigue, quality of road structure, there are a lot of elements that go into road crashes."

"We understand this needs to be balanced with the need for regional and country south Australians to actually get around, and we believe that balance is right at 110 kilometres an hour."

Lower limits save lives

However Craig Kloeden, from the Centre of Automotive Safety at the University of Adelaide, said reducing speed limits saved lives.

He said global research showed that lower limits reduced fatalities, injuries and crashes.

"In South Australia in 2003 there was about 1000 kilometres of road where the speed limit was dropped from 110kmph to 100, and we looked at crashes on those roads before and after those changes," he said.

"The average vehicle speed only went down by two kilometres an hour but we also saw a 27 per cent reduction in injury crashes."

Assault on regional electorates

Mr Knoll denied that his promise was a pitch for votes.

The Liberal Party faces a big challenge from the Nationals and Nick Xenophon's SA Best party in several regional seats.

The Nationals do not hold any seats, but are planning an assault on regional electorates in 2017.

"The Liberal Party has had a strong country representation since its inception, and in State Parliament at the moment all country seats — bar a couple — are held by the Liberal Party," Mr Knoll said.

"We have had a strong opposition to the reduction of speed limits for a long time,'' he said.

''This is an issue that comes up in our communities and this policy is a manifestation of the frustration brought to us by the respective communities."

The Liberal Party would not commit to spending money on regional road upgrades.

"We will definitely have more to say about what we are doing for regional road infrastructure," Mr Knoll said.

Road Safety Minister Chris Picton, and the former minister, Peter Malinauskas, have been contacted for comment.