A new section of Auckland motorway opened this year to speed up traffic has been found to have had the opposite effect.

The southeast-bound section of the Manukau connection, which links Auckland's Southwestern Motorway and Southern Motorway, was opened in August, cutting out 12 sets of traffic lights.

The motorway, part of the Western Ring Route project, was aimed at speeding up traffic times, but the joining of traffic from two motorways ended up increasing congestion, particularly during afternoon peak hours.

Now the New Zealand Transport Agency (NZTA) has decided to add ramp signals to try to improve traffic flow.

"We believe ramp signals are the most effective way to manage merging traffic," NZTA Auckland state highways manager Tommy Parker said.

"They have a double advantage of improving safety and smoothing the flow of traffic as the two motorways join."

Mr Parker said NZTA was looking at bringing forward work to widen the Southern Motorway south of Hill Rd in Manurewa from two to three lanes in each direction.

The ramp signs will be operational from next Monday.