A study says congestion charges shouldn't be seen as a revenue grab, and drivers will get on board if they see the results.

New research by business thinktank The New Zealand Initiative has studied global congestion systems and how they work, saying most commuters eventually come around to the idea.

Executive director Oliver Hartwich told Mike Hosking it needs to be designed correctly, with the right motives.

"If you're introducing a road pricing scheme with the the idea to make money, forget it. It's not a money-making machine, it is all about traffic, it iss all about making better use of existing road space."

He says that the charges are effective and drivers eventually notice the difference.

"If you just put a little 'c' on it, sometimes it takes five per cent of the drivers just to defer a trip to really make the traffic flow again."