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Auckland bus commuters soundly beaten to work by a Herald car on the Northern Busway's first big morning can at least congratulate themselves on doing the right thing by the planet.

The car's coverage of 15.8km from Campbells Bay in 37 minutes compared with a tortuous 52-minute trip suffered by a colleague in a crowded bus that turned up 10 minutes late.

This did not show the busway in a good light, even though only 2km of the bus route coincided with the new $300 million transit spine.

The 6.2km two-lane highway reserved just for buses fared somewhat better for a hybrid trip by a third Herald journalist, who used its full length while covering a more indirect route of 18.6km in 41 minutes.

That included a 4.7km leg by car to the Constellation Drive bus station, from where he caught a bus that did 13.9km in just 24 minutes down the uncluttered busway and across the harbour bridge.

But when some readers raised concerns that those time trials on February 4 did not explore environmental benefits of public transport, the Green Pages resolved to recycle the exercise through an unofficial carbon calculator.

The results showed that, based on fuel consumption alone, the carbon "footprint" of each of 50 bus passengers would have been six times smaller than those left by the single-occupant car.

Each bus passenger's fuel consumption would have released just 406g of carbon dioxide - the main "greenhouse" gas - into the atmosphere compared with the car driver's 2.4kg.

It was lucky for planetary health that he was using a 1500cc Hyundai pool vehicle, rather than a four-wheel-drive such as a 4-litre Ford Territory from the newspaper's executive carpark, likely to have pumped out 4.7kg for the one-way trip.

The environmental balance book would have looked even better for bus passengers if vehicle manufacturing costs were factored in.

According to the International Association of Public Transport, the amount of energy used to make an average car is 36 per cent higher for each "passenger kilometre" travelled than for a bus, although tiny particles spread by diesel fumes from heavy vehicles raise greater personal health concerns than petrol exhaust.

Diesel also has a relatively high carbon dioxide load, generating 2.605kg for every litre of fuel burned, compared with 91-octane petrol's output of 2.285kg.

But the Herald's calculations were based on average fuel ratings by either car manufacturers or bus operators and Auckland Regional Council environmental sustainability manager Gerda Kuschel says those do not take the special characteristics of each journey into account.

She said fuel-draining stop-and-start movements suffered by vehicles in congested traffic would put them at a particular disadvantage to buses enjoying constant speeds along their own dedicated highway.

The carbon equation was considerably less favourable for the combined bus-car trip, which was almost 3km longer than the other two.

There were only 15 passengers sharing the carbon footprint on the bus leg, just seven above a point at which driving a car would arguably have caused less global warming.

Although the Herald has yet to test a motorbike on the route, as demanded by some readers, some tests show a 150cc scooter could cover the route with a "footprint" of just 546g.