The speed limits apply on Estonia’s seasonal ice roads laid out across the frozen sea. For most of the year these routes are traversed by ferries. While the ice is normally thick enough to support vehicles, there is a particular danger between 25 and 40km/h, because the motion of the car can produce a wave in the ice resembling the bow wave of a ship. Carry on for too long in this speed range, and the wave becomes powerful enough to crack the ice, with disastrous consequences.
https://www.theguardian.com/ne...
