Days after the incident the Swiss government tightened rules governing car seats, approving plans to make them compulsory for children up to to 12 years old. They are presently only required for children aged up to seven.

The Touring Club Switzerland (TCS) said the death of the three-year-old in Zurich was a "really sad situation" but was the result of a highly unusual sequence of events.

According to police, the child unbuckled the seat, climbed across his two siblings sitting in the back seat, opened the car door, fell into the road and was hit by another car. The youngster was taken to hospital but died soon afterwards.

"I never heard of an accident like this before. The possibility that it could be repeated is really low – one in a billion," TCS child safety expert Arthur Keller told swissinfo.ch.

But it is a mistake to assume from the start that car seats guarantee safety, he explained.

Under a United Nations Economic Commission for Europe ruling, car seats must be manufactured in such a way that children as young as two can open the buckle in case of an accident or fire. The buckles must also be made to open in one swift movement. As a result Switzerland cannot sell car seats that only adults can open.

It is down to adults to ensure that the child keeps the car seat belt fastened, Keller said.

"For sure the responsibility lies with the driver or the parents. It is the parents' job to say, 'stop that, keep the buckle on'. If there is no reaction from the child then they have to stop the car safely."

Involve children

But incorrect use of seats is also a problem. TCS checks found that one in three children is not correctly buckled in or that the child seat has not been installed properly in the car.

Bearing in mind that most children dislike being put into the seats, he recommends involving children when choosing the seat and having them help pick the colour.

To improve safety child locks should also be used on car doors.

A Federal Roads Office proposal to make car seats compulsory for children up to age 12 or those who are under 1.5 metres tall was approved by cabinet on Wednesday.

The new measures will be effective from April 1, 2010 and are in line with neighbouring countries.

The initiative had the backing of the TCS, which for the past 30 years has advised parents to use the seats for children aged 12 and below.

The Swiss Council for Accident Prevention says the death of the three-year-old in Zurich underlines the importance of belting up properly. Its "Belt Up" campaign on seatbelts has highlighted child safety and says that drivers do "not always make children as safe as possible".

According to government statistics from 2007, one in six children is not buckled in at all.

Jessica Dacey, swissinfo.ch