A promise the Government's working as quickly as possible on roading projects - but an admission some will take a long time.

The Transport Minister says he's trying to make expectations clear.

Officials estimate petrol tax and road user charges would need to be hiked 70 percent - to deliver all the $56 billion Roads of National Significance - within two decades.

Chris Bishop says the Government is committed, but these are very big projects.

"So I'm just saying, it's about a long-term pipeline over many, many years."

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