The trial, carried out on a 12-kilometre (seven-mile) stretch of the A10 motorway between Spittal and Paternion in the southern province of Carinthia, is part of an initiative by Transportation Minister and vice-chancellor Hubert Gorbach to make speed limits more flexible.

But activists from the environmental group Greenpeace chained themselves together and blocked the motorway with a banner, three hours after the trial began at 5:00 am (0300 GMT), and compared Gorbach's plan to recklessly driving against traffic.

Rolf Holub, spokesman for the Green party, which later also staged a protest on the motorway, called the plan a danger to the environment, a security risk and illegal. Others also criticised the trial, arguing speed was the main cause of deadly accidents.

But according to Gorbach, since 40 percent of drivers drive over the limit anyway the initiative will actually be "a contribution to traffic safety," as high speed limits will only be implemented on closely monitored stretches of road.

Gorbach is a leader of Joerg Haider's far right BZOE party and his initiative has been tagged by opposition politicians as a campaign tool to attract votes ahead of general elections this autumn.

A "traffic impacting system" will monitor factors such as traffic flow, weather and road conditions and adapt the speed limit accordingly.

"In case of fog, bad visibility, slippery roads, increased traffic, accidents, jams, etc... the designated speed limit will be automatically dropped," according to a statement by the Austrian transportation ministry.

Conversely, if conditions are optimal, the limit could be raised.

Gorbach has also called for Tempo 160, as the project is called, to be accompanied by a number of other measures aimed at increasing road safety, including stricter penalties for speed limit violators and better training in driving schools.

Because of the protests on Tuesday, the speed limit on certain stretches of the A10 between Spittal and Paternion had to be brought under 80km/h (50 mph). The limit on Austrian motorways is normally 130km/h (80 mph).

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