Is it dangerous to drive on motorway with a crack in my windscreen?
It is not dangerous. As has already been said, windscreens are made from laminated glass and do not crack or break under wind pressure, only by being hit by something. Obviously there is the possibility of something being thrown up by a vehicle in front of you, then the screen may well break, but then that could happen even if you don’t have a crack already. Even if the screen breaks it will not shatter and you will not get covered in glass, it will actually stay in one piece where it is, but obviously your vision will be terribly obscured. If it is not in your field of vision then it may not HAVE to be replaced.