The number of over-70s who hold a UK driving licence has exceeded four million for the first time. But how dangerous are older drivers?

There are occasional very well-publicised stories about elderly drivers going the wrong way on a motorway or dual carriageway. Only last week an 84-year-old woman died after driving the wrong way on the A1 in Northumberland.

The RAC Foundation says the UK's oldest licensed driver is a 107-year-old woman, and there are 191 people over the age of 100 with a licence. They are among 4,018,900 people aged over 70 with full UK driving licences.

But how dangerous are older drivers?

The Department for Transport (DfT) says there is no evidence older drivers are more likely to cause an accident, and it has no plans to restrict licensing or mandate extra training on the basis of age.

The answer Drivers under the age of 20 have more fatal accidents than drivers over 75

Over 70s can struggle with high-speed junctions and slip roads

There is some evidence drivers over 80 are at increased risk

There were 10,974 accidents involving drivers over the age of 70 in 2011, says the DfT. That compares with 11,946 accidents involving 17-to-19-year-old drivers and 24,007 accidents involving 20-to-24-year-old drivers. Its statistics do not account for who caused the accident.

Figures also show that 46 drivers aged 16 to 19 died in an accident, while 173 drivers aged between 20 and 29 involved in an accident died. That compared with 59 deaths in drivers aged between 70 and 79 involved in an accident, and 52 over the age of 80.

The young-versus-old driver data is used by a number of road safety charities to argue elderly drivers don't pose the greatest danger behind the wheel.

"There's a stat that young drivers under the age of 24 have twice as many crashes as you'd expect, given the numbers on the road, and older drivers have half as many as you'd expect, given the number on the road," says Neil Greig, director of policy and research at the Institute of Advanced Motorists (IAM).

Research by the RAC Foundation suggests drivers aged 75 and over make up 6% of all licence holders but account for just 4.3% of all deaths and serious injuries. By contrast, drivers aged 16-20 make up just 2.5% of all drivers but 13% of those killed and seriously injured.

But although both charities believe older drivers are as safe as any other driver, there are some exceptions.

"In key locations such as high-speed junctions, high-speed roundabouts and slip roads onto motorways and dual carriageways - locations where drivers are required to look around quickly and make quick decisions - some drivers over the age of 70 struggle," says Greig.

Who, what, why? A part of BBC News Magazine, Who, What, Why? aims to answer questions behind the headlines

"Everywhere else - in towns, the countryside, the overtaking manoeuvre - they are as good as anybody else."

There is also some evidence to suggest drivers over the age of 80 are at an increased risk of accidents, according to Liz Box, head of research at the RAC Foundation. At that age infrequent driving is also a factor.

She says frailty is an issue with older drivers, so those involved in accidents are more likely to be injured.

At the moment, motorists over 70 must declare they are fit every three years, but they do not have to take a driving or medical exam.

Box says the RAC Foundation doesn't support compulsory testing for older drivers, as every individual is different.

"There are huge benefits to people driving, it helps them feel empowered. What we want people to do is see their optician regularly, and go to independent assessment centres if they are concerned," she says.

Greig agrees that "arbitrary age limits" aren't the answer.

"There's some evidence that suddenly stopping driving and a lack of mobility leads to depression, so we want to keep people independent for as long as possible. The key thing is for people to start preparing for it early so they have other options," he says.

Have you told an elderly relative to stop driving? How did you do it? How old were they? What prompted you to do it and what was their reaction?

Comment on Facebook or on Twitter using the hashtag #elderlydrivers.