"The ice road is opened in years when the winter is cold enough to freeze the sea strongly enough to carry cars," Raido Randmaa from Estonia's highways administration told AFP Friday.

Under a special law, Estonian authorities can give the green light to ice roads when the marine ice sheet becomes at least 25 cm (9.8 inches) thick, according to Randmaa.

"But we usually wait until it gets to 30 cm (11 inches) everywhere," he explains, adding the ice sheet was judged thick enough for traffic last week. The grand opening of the ice highway between the port of Rohukula in north-east Estonia and Hiiumaa island - population slightly over 10,000 - is always a big event for the islanders.

"The ice-bridge is always very popular and becomes very crowded on weekends," Randmaa explains.

Authorities are strict about safety on the ice, with patrols keeping watch around the clock.

"One of the main rules is that a new car is allowed on the ice road only every 1-2 minutes, and drivers should not get too close to the car ahead of them because otherwise the ice might crack," Randmaa told AFP.

Only Mother Nature knows how long Europe's longest ice road will stay open this year. In 2010, it lasted just 11 days in February.

"Our greatest enemy is not just warm weather but also the extreme cold that could start causing cracks inside the ice," Randmaa said.

Compared to the usual ferry service, the ice-road will cut travel time from Hiiumaa, Estonia's second largest island, to mainland Estonia by nearly two hours, making the trip only around half an hour.

Source: The Independent