The Transport Minister says it's hard to know what a road will cost until construction begins.

Chris Bishop has released a new phased timeline for the 17 transport projects on the Government's to do list - putting five on the back burner. 

The whole 17 roads are expected to come in at roughly $56 billion. 

Bishop says these projects will be completed in phases, and there's no market capacity to get all of these built at the same time. 

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