The number of young people who have been killed in car crashes this year has soared, as experts warn P-plate drivers their inexperience on the road is one of the most likely causes.

Just two days after three Victorian teenagers died in a crash in the state's outer east, figures from the Transport Accident Commission show that 15 people aged 16 to 17 have been killed so far this year, compared with two in 2014.

The five-year average of deaths in this age group is five.

TAC senior manager, road safety, Samantha Cockfield​ said the jump in young fatalities occurred in the past six months and, so far, there was no explanation for the rise.

"Young drivers are still developing and this is not that they don't understand the risk – they're well educated – but when faced with social situations, with peers, making good choices is hard," she said.

"I don't think we can explain the numbers but what we do know is the underlying issues. We need to encourage good decision making and help young drivers understand the sort of issues they'll face."

Two 17-year-old girls, Sharn Walker and Felisha Allen, and 18-year-old Jason Breakwell were killed after the car they were in crashed into a tree on an unsealed road in Avonsleigh on Tuesday. Jake Mitchell, 19, was injured and was released from hospital on Wednesday.

The car, a Ford XR6, was hardly recognisable because of the force of the impact.

It is understood Major Collision Investigation Unit detectives are looking at speed on the 50km/h road as a major factor, coupled with driver inexperience.

TAC figures show first-year drivers are almost four times more likely to suffer serious injuries or die in a crash than older drivers.

VicRoads introduced a graduated P-plate system in 2007, with additional restrictions imposed for the first 12 months. Probationary P1 drivers are not allowed to carry more than one passenger aged between 16 and 22.

Victoria Police did not respond to questions about how often this rule was breached.

Associate Professor Stuart Newstead, from Monash University's Accident Research Centre, said the passenger rule had been very effective.

"You don't get that peer pressure to do anything stupid; it's an effective component to cut fatalities and injuries," he said.

However, young drivers still struggled against their natural immaturity and inexperience, which affected their ability to judge situations and how to avoid risks, he said.

In 2014, 20 per cent of drivers killed were aged between 18 and 25. This is despite this age group representing just 14 per cent of all licence holders.

Of the 24 young drivers killed last year, 71 per cent were killed on country roads.

A recent Australian Transport Safety Bureau report has found that lack of experience, limited ability and judgment, underestimating risks, deliberate risk-taking behaviours and use of alcohol and drugs are the main reasons for increased risks for young drivers.