BRUSSELS, Dec 10 (Reuters) - European Commission President Ursula von der Leyen said on Thursday she had laid out measures to cope if Britain and the European Union fail to agree a trade deal by Dec. 31. The measures would allow smooth air and road travel for the next six months, among other measures, von der Leyen said. “Our responsibility is to be prepared for all eventualities,” she said in a statement posted on Twitter. (Reporting by Brussels newsroom; Editing by Alison Williams)