A woman accused of hitting and killing a cyclist says she didn’t see the grandfather because of shadows on the road.

Bill James died when he was thrown from his bike after being struck from behind by a car in Coolup in June, 2017.

Brooke Kau was behind the wheel of that car and says shadows on the road meant she didn't see the 76-year-old.

Prosecutors claim she wasn't paying attention when she slammed into the cyclist.

They say Ms Kau had been drinking with friends the night before.

Tests showed she didn't have any alcohol in her system the next morning but the state says it's likely she was distracted by her phone.

Just before 6.30am she messaged a friend saying: "I'm not going to be able to drive for a bit.”

Replying to her friend over an hour later she said: "I’m vomiting in the servo!"

The prosecution says the final message Brooke Kau sent from her Samsung galaxy mobile phone was at 8.18am, just moments before the deadly crash happened.

Ms Kau maintains she pulled over to send that message.

The trial is set to continue for four days.