A legal expert says it's important the opposition parties have their say on the traffic light system

National and Act have raised concerns about the speed and lack of consultation around the system, which is due to begin on Friday week.

The legislation's expected to be passed under urgency in Parliament this week.

Waikato University law professor Alexander Gillespie says the normal Select Committee procedure isn't possible because there's limited time.

But he says democracy requires as much input from the opposition as possible.

He says the more chance others have to look at the proposed legislation, the fullest possible debate we can have in a short time.