Too fast? A 100kmh speed limit on open roads without median barriers is asking for trouble, a new report suggests.

Coming to a rural road near you - a 70kmh speed limit?

A landmark report by an inter-governmental organisation with 59 member countries - including New Zealand - is recommending that rural roads that don't have median barriers should have their speed limits lowered to 70kmh.

Taken literally, that would mean that almost NZ's entire roading network, state highways included, would need to have their speed limits reduced to 70kmh. Only those section of motorways with median barriers would be allowed to retain their 100kmh limit.

CALLUM MCGILLIVRAY/STUFF Too fast? A 100kmh speed limit on open roads without median barriers is asking for trouble, a new report suggests.

Not only that, but the report out of Paris by the International Transport Forum also recommends a 30 kmh speed limit for built-up and residential urban areas where vehicles and vulnerable road users such as cyclists and pedestrians share the same space, and 50kmh for other urban areas with intersections and high risk of side collisions.

READ MORE:

* New Zealand's first 110kmh roads get green light

* The slow road out of town: speed review set to cut limits

* Feedback sought on speed changes

Once again that would have a major impact in NZ, where urban speed limits are currently 50 kmh except in inner-city areas and around places such as schools

Joseph Johnson/Stuff Every 1 per cent increase in average vehicle speed results in a bigger increase in frequency of crashes, says the report

It's all the result of a big study that has examined how road safety performance in 10 selected countries changed after they changed speed limits or introduced automatic speed cameras on a large scale.

The International Transport Forum acts as a think tank for transport policy, and it organises an annual summit of transport ministers from governments around the world. This latest report, entitled Speed and Crash Risk, has been released prior to the next summit, which will be held in Leipzig in Germany next month, and which will discuss road safety.

The report says studies show that every 1 per cent increase in average vehicle speed results in a 2 per cent increase in the frequency of injury crashes, a 3 per cent increase in the frequency of severe crashes, and a 4 per cent increase in the frequency of fatal crashes.

Supplied Maybe New Zealand's entire rural roading network should have a 70kmh speed limit?

"Thus, reducing speed by a few kmh can greatly reduce the risks of and severity of crashes," says the report.

"Lower driving speeds also benefit quality of life, especially in urban areas as the reduction in speed mitigates air pollution, greenhouse gas emissions, fuel consumption, and noise."

The report admits that speed is important in modern-day transport. It directly affects the mobility of people and goods travelling from one location to another. In today's modern life, people want a high degree of mobility, and to be able to travel fast by air, rail and road has become a requirement of our societies.

JACKIE NORMAN How would you like trundling down New Zealand's iconic rural highways at 70kmh?

But excessive speed is now a major problem in all motorised countries, it adds. An estimate in Norway shows that if all motorists drove below speed limits, the number of fatalities would reduce by 20 per cent.

Speed enforcement is therefore a major challenge for all governments.

"The severity of a crash follows the laws of physics," says the report. "At higher speeds the kinetic energy released in a crash increases with the square of the speed, and the changes in speed experienced by those struck by or occupying the vehicles involved increase with speed.

Supplied Speed is important in modern-day transport, admits the International Transport Forum.

"The increase in crash risk is explained by the fact that when speed increases, the time to react is shorter and manoeuvrability is smaller. In traffic, drivers on average need about one second to react to an unexpected event and choose an adequate response - this is called the reaction time.

"The higher the driving speed, the longer the distance you cover during this reaction time, reducing the opportunity to avoid a crash."

With that as background, the design of a road system and the speed limits set for it must consider the forces the human body can tolerate and survive in a crash, says the report.

It recommends reducing speeds on roads as well as speed differences between vehicles, and working to what is known as a Safe System which would set speed limits low enough to ensure crash severity can be survivable without dramatic consequences.

The report also recommends the use of automatic enforcement of speed limits - this system monitors average speeds over sections of roads.