Driving children to school does not turn them into couch potatoes, UK research suggests.
Walking to school did little to activity levels
Whether a child walked to school or was driven made no difference to their overall activity levels, scientists at Derriford Hospital in Plymouth found.
The additional activity of the average six minute walk to and from school each day made up only 2% of the children's total weekly activity.
The findings appear on the British Medical Journal website bmj.com.
Obesity in children is on the increase in the UK.
School runs, along with television and computer games, have been blamed for sedentary lifestyles and the
increasing childhood weight trends.
Compared with 1986 when 16% of children were driven to school, the figure doubled in 1998 to 30%.
The government has introduced a number of initiatives to combat the obesity problem, including encouraging parents to walk their children to school.
But Professor Terry Wilkin and colleagues have found walking to school does little to increase the overall activity of primary school children.
They asked 154 boys and 121 girls in their first year of primary school to wear monitors that measured activity during their waking hours for five consecutive school days and a weekend.
Twice as many boys and girls walked to school as were driven by car.
Although the children who walked the average half kilometre journey each day did do more activity, about six minutes more per day, the total difference over a week was trivial.
The additional activity totted up by walking rather than travelling to school in a car amounted to only 2% of the children's total weekly activity.
There may be other benefits from walking children to school, but physical activity does not appear to be one of them, say the researchers.
Co-author Brad Metcalf said: "There is an assumption that walking to school increases levels of physical activity.
"But actually, it's such a small time of the day, about six minutes, that the difference is lost when you look at the whole week.
"It's easy to blame school runs for obesity, but if you want to increase physical activity maybe you need some parental responsibility to take children to sports clubs and get them active in other ways.
"Just walking them to school is not going to increase physical activity or stem the rise in obesity."
He said the government should take note of the findings and change its strategy to encourage children to exercise more.
"The government is spending money on trying to encourage primary school children to walk to school. Instead it should be encouraging perhaps more PE time and after school activities," he said.
Dr Ian Campbell from the National Obesity Forum said: "Schools are often subject to undue attention as being the source of activity amongst children.
"It's important to encourage physical activity on the way to school and in school because it sends out the right message that it's fun and healthy.
"But it must never be seen as a get out for parents and others in charge of children to encourage them to be active in the home.
"Kids spend far more time at home than at school," he said.