Safety fears over tourist drivers on New Zealand roads became so heated this summer that even Prime Minister John Key got involved.

He urged calm after people started taking car keys from overseas drivers to keep them off the roads.

There have been numerous reports of tourists involved in crashes on Kiwi roads this summer.

One of the most high-profile involved the death of 5-year-old Ruby Marris in a crash near Moeraki in February.

A Chinese driver is facing dangerous driving charges over the crash.

Read more:

- Tourists at fault in 1 in 12 South Island crashes

- Son of man killed by Chinese driver makes plea for safer roads

The death of a child is always deeply emotive but what can crash data tell us about the relative dangers of overseas drivers?

To try to find out, we compared the percentage of fatal and serious crashes where overseas drivers were at fault with the percentage of all New Zealand vehicles that were rental cars driven by tourists.

The analysis found that overseas drivers show up in crash numbers during the tourist season at about nine times the rate they show up on the road.

There are limitations to the data. It is difficult to estimate the percentage of vehicles in a particular region during a particular month that are rental cars driven by tourists.

For example, to roughly estimate the percentage of tourist drivers on Canterbury roads in January 2013, visitor figures, local fleet numbers and tourist surveys conducted by the Government every year were used.

Visitor surveys show that about 1 million people visited Canterbury in 2013.

About 12.6 per cent of all tourists in 2013 drove rental cars, according to government visitor surveys. That means about 126,000 of those 1 million Canterbury visitors drove rental cars. About 10 per cent of international visitors come to New Zealand in January, which means there were about 12,600 tourists in rental cars on Canterbury roads in January.

This was then compared to the 500,000 vehicles on the Canterbury roads in 2013 - figures provided by the Ministry of Transport. This means that tourists in rental cars made up about 2.5 per cent of all local vehicles in January 2013. An eight-year average for every January between 2006 and 2013 was then compared with the percentage of crashes in which overseas drivers were at fault.

One limitation here is that we are comparing an estimated percentage of tourists in rental cars to overseas drivers, which in crash data also includes tourists, students, visitors, and immigrants driving on an overseas licence.

It is not apples for apples but does give us a rough idea.

The percentage of fatal and serious crashes where overseas drivers were involved or at fault was calculated from data supplied by the NZ Transport Agency for 2006-13.

A comparison between the two sets of data shows that tourists in rental cars make up a small percentage of all the vehicles on New Zealand roads in any month, but overseas drivers are involved in a much larger percentage of fatal and serious crashes.

In the Canterbury example, tourist drivers make up about 2.6 per cent of road traffic on average for January, but are at fault in about 7 per cent of fatal and serious crashes.

Across New Zealand, February is the worst month. Tourists in rental cars make up less than 1 per cent of all vehicles on the road on average for February but overseas drivers are at fault or part fault in nearly 7 per cent of crashes in February from 2006 to 2013.

For the busiest tourist month of January, the worst tourist crash rate was in the Bay of Plenty, where overseas drivers were at fault in 5.9 per cent of crashes but tourists in rental cars made up about 1 per cent of vehicles.