Despite motorists having a $630 million expressway, new figures show the rush-hour crawl along State Highway 1 between Wellington and the Kāpiti Coast is moving slower than before the new road was built.

A new Greater Wellington Regional Council report shows the average travel speed on that route during the morning peak is slower by 4.5 kilometres an hour to a year ago, and is the slowest it has been in four years.

Data recorded during March found the average travel speed from Waikanae to Wellington Airport during the morning peak dropped from 48.8kmh in 2016 to 44.3kmh this year.

MONIQUE FORD/STUFF A new report has found that travel speeds in and out of Wellington during peak times on six key routes have largely gotten worse over the past year.

Elsewhere across the region, average travel speeds have also dropped noticeably during rush hour between Petone and Wainuiomata in Lower Hutt, on State Highway 58 between Paremata and Haywards Rd, and between the Wellington suburb of Karori and Bowen St near Parliament in the CBD.

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The report measured the average travel speeds along six "strategic routes" in and out of the capital during the morningand evening peak. Average speeds on all but one of those routes have fallen since 2014.

MAARTEN HOLL/STUFF The $630m Kāpiti expressway has done little to lift travel speeds from Waikanae to Wellington Airport on State Highway 1 since it opened in February.

Luke Troy, the council's general manager of strategy, said it was difficult to pinpoint why traffic was moving slower.

"There is wide conjecture, but it is obviously indicative of increased congestion. What is causing that congestion is hard to tell."

Because the figures were only averages taken in the month of March, factors such as road works and other local events could impact data in any one year, Troy said.

CAMERON BURNELL/STUFF.CO.NZ Reporter Joel Maxwell takes a Wednesday morning commute through the Kapiti Coast on the old State Highway 1 in mid-February.

Net migration has also increased by 1.8 per cent across the Wellington region in the past year, while the number of vehicle kilometres travelled has risen by 5.7 per cent between 2014 and 2016.

"There are more people driving, and people driving are doing more trips," Troy said.

The four-lane, $630m first section of the Kāpiti expressway north of Wellington, from Mackays Crossing to Peka Peka, opened in February before the most recent data was taken.

ROBERT KITCHIN/FAIRFAX NZ Maxwell drives from Peka Peka to Poplar Ave on the new expressway, at the same time on a Wednesday morning.

It initially created a bottleneck at the point where the expressway connected to the old two-lane SH1, lengthening travel times during the morning rush between Raumati and Mackays Crossing.

Mark Owen, regional transport systems manager for the New Zealand Transport Agency, said the full benefits of the Kāpiti expressway would not be realised until other roading projects north of Wellington were completed.

They included the second section of the expressway, from Peka Peka to Otaki, and the $850m Transmission Gully motorway.

STUFF The average speed at which traffic crawls between Karori to Bowen St in central Wellington during the morning rush is just 18.5kmh.

Disruptions such as weather events, slips, roadworks and crashes also impact on travel times, and the agency was working hard to increase resilience across the transport network, Owen said.

The situation was even worse in Lower Hutt, where the average travel speed from Petone to Wainuiomata in the evening peak dropped from 41.9kmh to 31.8kmh.

Ken Laban, regional councillor for Lower Hutt, said traffic in the area was a "nightmare".

JARED NICOLL/STUFF The journey between Petone and Wainuiomata, via the Wainuiomata hill road, during the evening rush is now 10.1kmh slower than it was a year ago.

There were 15,000 cars on The Esplanade in Petone every day, with over 10,000 coming from Wainuiomata, he said.

"We need to start having serious conversations; not about roads, but about reducing the number of vehicles on the road."

In Wellington's western suburbs, the average travel speed during the morning rush from Karori to Bowen St dropped from 25kmh in 2016 to 18.5kmh this year. The return journey in the evening was down from 26.8kmh to 25.1kmh.

ROB KITCHIN/STUFF The rush hour journey in and out of Wellington from the Hutt Valley, via SH2, has gotten slightly faster, particularly in the morning.

​Wellington city councillor Andy Foster said, with forecast population growth of up to 80,000 in the CBD over the next 10 years, it would be a challenge to even keep traffic moving that quickly.

"It's going to require some significant shift in travel behaviour. At 18.5kmh, a good runner is going to beat that."

But it was not all bad news for motorists. The average travel speed between Upper Hutt and Wellington's CBD on SH2 picked up in both the morning and evening.

The average travel speed from Wainuiomata to Petone during the morning rush also increased by 3.6kmh.

The report also showed transport-generated carbon emissions across the Wellington region have risen by 6 per cent since 2013, to 1123 kilotonnes.

The regional council has targeted a 10 per cent annual reduction in carbon emissions by 2025.

Troy said the increase was probably also due to increased vehicle use and trips, as well as an ageing vehicle fleet.

AVERAGE TRAVEL SPEEDS

Waikanae and Wellington Airport (SH1)

Weekday morning peak (inbound) - 44.3kmh (4.5kmh slower than a year ago)

Afternoon peak (outbound) - 38.9kmh (0.6kmh faster)

Upper Hutt and Wellington CBD (SH2)

Morning peak - 38kmh (4.8kmh faster)

Afternoon peak - 45.7kmh (1.2kmh faster)

Paremata and Haywards Rd (SH58)

Morning peak - 52.8kmh (3.1kmh slower)

Afternoon peak - 58.1kmh (1.5kmh slower)

Karori and Bowen St

Morning peak - 18.5kmh (6.5kmh slower)

Afternoon peak - 25.1kmh (1.7kmh slower)

Island Bay and Wellington Railway Station

Morning peak - 22.8kmh (01.kmh slower)

Afternoon peak - 22.3kmh (0.6kmh faster)

Wainuiomata and Petone

Morning peak - 20.9kmh (3.6kmh faster)

Afternoon peak - 31.8kmh (10.1kmh slower)

AM peak: 7am to 9am

PM peak: 4pm to 6pm