A driver sent and received more than 20 text messages on the journey which ended in her hitting and killing a stationary motorist, a court has heard.
Philippa Curtis, of Suffolk, hit the back of Victoria McBryde's car at 70mph (113km/h) on the M40 in Oxfordshire.
Ms McBryde, 24, from Northamptonshire, suffered a fatal brain injury.
Ms Curtis, 21, told Oxford Crown Court she thought you could use a phone when driving "in the right conditions". She denies death by dangerous driving.
Ms Curtis, of Icklingham, in Bury St Edmunds, had been on her way to stay with her boyfriend in Oxford.
She said she would have probably "slowed down" to send the messages on 20 November last year.
The court heard Curtis had made the two-hour journey to Oxford on a dark evening when the roads were wet and after a shift at a restaurant in Suffolk.
The waitress, who told the court she had been "hyper" as she set off at 2100 GMT, made various calls and sent text messages to a number of friends using predictive text on her flip-top phone.
As she arrived in Oxford she made a call to her boyfriend which, she claims, did not go through.
She then dialled a taxi firm so she could arrange an onward journey from a park-and-ride.
But shortly after making the call, she hit a Peugeot 106 belonging to Victoria McBryde, who had stopped to deal with a burst tyre.
Ms Mcbryde's car was forced off the carriageway and ended up on the concrete.
Ms Curtis, who only suffered an arm injury, spun into oncoming traffic hitting two more vehicles, a white van and an Asda lorry, the court heard.
But giving evidence, the 21-year-old said she had felt there were times when using a phone while driving was acceptable.
She told the court: "I didn't think I should be chatting away when manoeuvring roundabouts, but in the right conditions..."
Prosecutor Alan Blake replied: "In the right conditions on the M40 at 70mph?"
She added: "I probably would have slowed down."
She also told the court she could send and receive messages without taking her eyes off the road.
The trial continues.