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Several hours later the teen, who was on a learner licence, crossed the centre line, crashed into another car and killed not only himself but also his back seat passenger Burgundy-Rose Marie Brown.
The dangers of driving on little sleep have been highlighted by Coroner Sue Johnson in a recently released report into the tragic deaths of the two 16-year-olds in March 2018.
The day before the crash, Dexter had finished a night shift at a factory. He was determined to save money to buy a car but his mother had told him he must get his restricted licence first. He held only a learner's licence.
However, the details about the trip were not true. Dexter had arranged to go with friends to Nelson to buy a car he had seen online. He slept for most of the trip, and while they waited that night for the owner of the Nissan he was buying, Dexter shared a joint with a friend. He bought the car, and the group headed back for Christchurch during the night.
At the first rest break in Murchison, his friends in the other car told him that he had been drifting in his lane and his headlights had stayed on high-beam when traffic was approaching. He said he was tired, but wanted to continue. The friends repeated the warning at the next stop at Springs Junction.
At a stop at Culverden, Burgundy-Rose got into the back seat of the Nissan and fell asleep. Dexter continued driving, staying awake by listening to music and talking to his front-seat passenger, who later fell asleep.
At 6.20am, two motorists in the northbound lane on State Highway 1 near Amberley crested a rise and saw headlights approaching in their lane. The vehicle appeared to make a small correction just before impact. The front-seat passenger had woken and grabbed the steering wheel just before the crash.
Paramedics attended but Dexter and Burgundy-Rose had both died, and three others in the vehicles were injured.
Large funerals were held for the two popular teenagers. Dexter was a car enthusiast who wanted to be a diesel mechanic; Burgundy-Rose had dreamed of being a flight attendant to see the world.
Coroner Sue Johnson's inquest findings highlight the dangers of driving tired, a message she wanted relayed through the media to raise awareness of the issue.
She also flagged issues related to consuming cannabis, though the crash would have happened hours after the young driver had shared a joint with a friend – well beyond the three hours that THC from cannabis remains in the system and causes impairment.
Coroner Johnson said the police and public agencies had consistently highlighted the dangers of driving while fatigued or after having consumed cannabis – a combination that greatly increased the chances of an accident.
THC from cannabis had been shown to impair performance, cause sleepiness, and "impair sustained vigilance", the effects remaining for up to three hours.
Tiredness reduced attentiveness and alertness to dangers, slowed reaction time, and led to poor lane tracking. It could cause dangerous "microsleep". Coroner Johnson said: "If this happens while driving it can cost you your life."
Meanwhile findings have also been released into the deaths of two teens in Hawera in March 2019 that also involved an inexperienced driver in a new car.
William John Wallace and Tadhg Lewis Hugh McColl were both 18 when the vehicle Wallace was driving left the road and rolled several times.
The Serious Crash Unit said Wallace was driving at between 118 and 146km/h in a 70km/h area. Williams had only bought the vehicle that day, and was driving with a passenger in breach of his restricted licence.
Coroner Mary-Anne Borrowdale said drivers should drive defensively, avoid aggressive driving, and keep their speed down.
"This is not the first coronial case in which I have dealt with a fatal crash caused by a young driver who has only just acquired a new vehicle," she said in her written decision.
"It takes time to become accustomed to the characteristics of a new car. Owners of new cars should – whatever the permissions on their licence – avoid carrying passengers until they have had time to adapt their driving style to the new vehicle."
- By David Clarkson, Open Justice multimedia journalist