Drivers in Wellington can now use the newly built Arras Tunnel, running underneath the planned National War Memorial Park.
The tunnel replaces a diversion that had been in place on Buckle Street, which was closed last night when the new route opened.
The underpass was developed so the Memorial Park could be built on top of it.
A spokesperson for the project, Brian Aspen, said drivers should take care when using the tunnel.
"To be aware it's changing, to pay attention because it will be slightly different, if they pay attention and follow the road signs they'll find themselves smoothly entering the tunnel."
Mr Aspen said the tunnel could result in slightly shorter travel times.
The tunnel took its name from the French town of Arras where hundreds of New Zealanders built an underground cave network during the First World War.
The underpass was opened by the Minister of Arts, Culture and Heritage, Chris Finlayson, at on Saturday. People able to walk through until it was shut again at about 2pm Saturday.
As part of the opening, there was a wreath-laying ceremony to recognise the New Zealand tunnellers' work in Arras.