OXFORD, England — Inside the imposing British Crown Court here, Phillipa Curtis, 22, and her parents cried as she was remanded for 21 months to a high-security women’s prison, for killing someone much like herself. The victim was Victoria McBryde, an up-and-coming university-trained fashion designer.

Ms. Curtis had plowed her Peugeot into the rear end of Ms. McBryde’s neon yellow Fiat, which had broken down on the A40 Motorway, killing Ms. McBryde, 24, instantly.

The crash might once have been written off as a tragic accident. Ms. Curtis’s alcohol level was zero. But her phone, which had flown onto the road and was handed to the police by a witness, told a story that — under new British sentencing guidelines — would send its owner to jail.

In the hour before the crash, she had exchanged nearly two dozen messages with at least five friends, most concerning her encounter with a celebrity singer she had served at the restaurant where she worked.

They are filled with the mangled spellings and abbreviations that typify the new lingua franca of the young. “LOL did you sing to her?” a friend asks. Ms. Curtis replies by typing in an expletive and adding, “I sang the wrong song.” A last incoming message, never opened, came in seconds before the accident.

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