Transit New Zealand says it is prepared to reduce open-road speed limits to 80 km/h on stretches of highway where travelling at 100 km/h may be unsafe.

New chief executive Rick van Barneveld acknowledges there are "certainly some" parts of the country's 10,800km state highway network which were designed for a maximum speed of 80 km/h before the limit was raised in 1985.

He said many were safe to drive on at 100 km/h, and others were being improved with guard rails and by widening the shoulders.

But where there was too little opportunity for motorists to recover from driving errors, Transit under his leadership would not hesitate to reduce the speed limit to 80 km/h.

This has already been done for several years along a 10km coastal section of State Highway 1 north of Wellington.

Mr van Barneveld's comments followed criticism of roading standards from international guests at a traffic engineering conference in Napier this week.

Swedish safety expert Thomas Carlson told the Weekend Herald his concern was directed at the standard of roading internationally, and not specifically at New Zealand.

He said the world's roading system in general was so dangerous it would be prohibited if invented today, and questioned the value of making vehicles safer without also improving roads.

"If you have a five-star car on a two-star road it doesn't help you if you run into a lamp-post."

But although diplomatic in his observations after a drive from Hamilton to Auckland - apart from expressing concern at the number of potentially hazardous objects along the side of the highway - an Australian guest at the conference was more direct.

Professor Ian Johnston, of Monash University's Accident Research Centre, said that although he had no problem reducing his speed to match the conditions of New Zealand roads, "I do get frightened by the traffic around me."

Speeds prevailing on many New Zealand roads were far higher than the level of safety afforded by the infrastructure, especially with police tolerance of speeds of 10 km/h above official limits.

He believed a 90 km/h limit would be more suitable for most two-lane rural roads, especially those with challenging terrain, trees alongside them, and a lack of sealed shoulders.

Mr van Barneveld denied that New Zealand roads in general were outdated.

His national network operations manager, Dave Bates, said a country of four million people would probably never have enough money for all the roading improvements it would like over such a large network.

But he said there was plenty Transit was doing to keep motorists from crashing off roads or, failing that, to create roadside buffer zones such as by removing trees or erecting guard rails around hazards to lessen the impact of collisions.

Mr Bates accepted that some curves were designed for an 80 km/h limit, which was the reason for signposts with lower recommended speeds.

Land Transport Safety Authority spokesman Andy Knackstedt said some New Zealand roads designed for an 80 km/h limit were "not very forgiving" in view of the challenging terrain they crossed.

However, he said Transit had made considerable improvements.

These included removing thousands of "black spots" identified by his agency since 1985 in a crash reduction study, an exercise calculated to have reduced smashes by 34 per cent at those sites at an estimated annual "social cost" saving of almost $3 billion .

Associate Transport Minister Harry Duynhoven acknowledged some roads had simply been widened over the years from goat tracks, and were designed for a maximum 80 km/h.

However, he pointed to extra Government money allocated for engineering safety improvements.

This included an extra $47 million announced in October for two years of minor safety works, more than double the amount earmarked earlier by funding agency Transfund.