The Waterview motorway connection is receiving enthusiastic reviews from motorists in every respect but one. Why, a petition of 4000 people is asking, does a grand, four-lane stretch of motorway with no tight corners require them to reduce speed from 100 to 80kph? The connection is, of course, partly underground, in twin tunnels that are longer than any others on our motorways. But they are not unusually long by international standards.

The New Zealand Transport Agency's system design manager, Brett Gliddon, says the lower speed limit accords with international best practice but many New Zealanders who have used urban motorway tunnels overseas will be surprised to hear it. Traffic generally flows as smoothly through underground sections with no perceptible drop in speed. Not so on Auckland's northwestern and southwestern motorways now.

Where is was previously possible to drive from West Auckland into the city at 100kph all the way in off-peak traffic, now signs order speed to be reduced to 80 from the Patiki Rd off-ramp until Western Springs, and as far as St Lukes on the southwestern motorway. These sections are all fine, wide carriageways, including the widened causeway to Te Atatu, and the camber is perfect for a comfortable ride at ordinary motorway speed.

Inside the tunnels, the road is three lanes wide, well lit and gently curved at each end for a safe transition into daylight. Yet signs insist traffic must not exceeds 80kph and must not change lanes. It gives travellers plenty of time to admire the engineering but, having done that, they also have a long time to wonder why the rules are so fussy and the pace so funereal.

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Gliddon explains that the tunnels and their approaches are "complex systems" with high traffic volumes and significant lane changing and merging required. But he adds that NZTA is "monitoring the ongoing management of speeds through this area and whether the current limit is the most appropriate to ensure safe and efficient journeys." So perhaps the agency will relax before too long.

The delay in the tunnels' opening and the installation of light controls, as well as the lower speed limit, all suggest our motorway managers are extremely nervous about road accidents underground. One reason for that may be a fatal accident in Melbourne's Burnley tunnel in March, just before Waterview was due to open. The 80kph limit in the Citylink tunnels was extended back to the Westgate flyover after the accident.

But Sydney does not seem to have followed suit. Its traffic authorities post some sensible tunnel driving precautions - remove sunglasses, turn headlights on, avoid changing lanes, keep a safe distance from the vehicle in front, read electronic message boards and leave the radio on - but no mention of lower speed.

The whole purpose of motorways is to provide a fast arterial route uninterrupted by cross-roads, traffic lights or varied speed restrictions. When the previous Labour Government decided the Waterview connection should be underground, no mention was made of a cost in travelling convenience.

It would have been a splendid motorway above ground and it needs to be just as good below ground. Let us hope the needless speed restriction is soon removed.