A pre-election promise of a $75 million transport interchange at Darlington in Adelaide has been put off because of a lack of federal money.

The interchange would have connected the planned duplication of the Southern Expressway with South Road.

Instead, Transport Department CEO Rod Hook said traffic lights would link the Expressway and South Road.

But he told a State Parliamentary hearing the $370 million Expressway duplication remained on track.

The SA Opposition said before the 2010 state election the Government had vowed it made no sense to duplicate the Southern Expressway without spending $75 million on a Darlington interchange.

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Both major parties had promised to duplicate the one-way road in Adelaide's south in their state election campaigns.

Labor has promised the duplication work will start late this year and be finished by the middle of 2014.

With the revelation of changes to the project, Transport Minister Patrick Conlon has rejected any suggestion the Government is breaking an election promise or that it misled voters, saying the transport artery is still being duplicated, as promised.