Fear of strangers or hurtling traffic might be the obvious answer.

But many parents are worried that other parents, friends and teachers will disapprove, and judge them, according to a comprehensive survey of Victorian parents from the Judith Lumley Centre at La Trobe University.

Almost 1800 parents of children aged nine to 15 were interviewed on the phone about the social, environmental and other factors that made them think twice about letting their children walk to school alone, or play in the neighbourhood without an adult.

And the perceived disapproval of family and friends was one of the strongest factors, says La Trobe researcher Shannon Bennetts.

"Parents are taking cues from other families in their school as a reference point," says Dr Bennetts.

"Social norms and community norms are shaping parent's decisions about letting their children be independently mobile."

This finding was no great surprise to Erin Feeney, whose three children Henry 11, Tom, 10 and Annabel, eight, walk about a kilometre from their home to Balwyn Primary each day alone.

Their journey to school includes crossing a busy intersection.

The children began making their own way to school when Henry was nine, because Ms Feeney had to drop Annabel at an after-school class and didn't want to pay for babysitting.

She "trained" them initially, walking behind them to observe how they reacted to traffic, and she still does not let them use a ball or scooter to prevent distractions.

They also have to ring by 4pm when they get home, and the neighbours have a key.

Some fellow parents were very positive, and offered to keep an eye on the children en route and report back if they weren't being sensible.   

But others told her she was doing the wrong thing.

"After it happened a few times I felt I had to justify it by explaining the training we had done with them," Ms Feeney says.

"I shouldn't have to justify it."

"But I also had parents say to me that [not letting their own children walk to school by themselves] was more about them, about not being able to let go."

One in four Australian children is now overweight or obese.

Strategies to encourage the state's increasingly-sedentary children to walk to school have focused on forming "walking buses", changing traffic and pavement infrastructure and signage to encourage walking.

But Dr Bennetts said strategies that focus on the built environment have been shown to increase children's independent mobility in the short term, but don't create sustained change.

It might be more effective to focus on the social factors, she said.

The research found parents with daughters and those from culturally and linguistically diverse backgrounds were particularly concerned about strangers harming their children.

This reflects community awareness of violence against women and cultural differences about what's appropriate for children, researchers said.

Giving a mobile phone to a child aged 10 to 13 – the transition age between primary and secondary school – meant parents were less fearful about strangers.

Ms Feeney says her children spend their walk talking to each other, and most of the feedback she gets from other parents is about how close her children seem.

"They walk along, heads next to each other, chatting away," she says.

The Age