When the official holiday period finished at 6am yesterday there had been 15 fatal crashes on New Zealand roads since the Friday before Christmas.

This was three more than last Christmas-New Year and it is in line with a rising annual road toll since 2013. Until a few years ago the toll was on a declining trend. What has happened?

It is hard to draw general lessons from road accidents. No two are exactly the same. Weather and driving conditions and drivers' behaviour are different, even between accidents on the same stretch of road.

The best that police and safety researchers can do is count the number in which speed was a factor, or alcohol or, these holidays, not wearing seatbelts.

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Stronger law enforcement and road safety information campaigns based on this research are probably the reason the toll declined for a few years but their impact might always be temporary.

The death toll rose from 293 in 2014 to 319 in 2015, to 327 in the year just gone, and that happened despite speed cameras' tolerance level being set far lower. Many drivers can attest to the large number of tickets they received from the same camera, sited on a downhill stretch of an arterial road.

Law enforcement like that merely teaches regular commuters where the cameras are, puts their focus on the speedometer rather than the road, and gives rise to the false but familiar conclusion that these campaigns are designed for revenue collection as well as road safety.

Readers discussing the holiday toll on this page have suggested the solutions lie in law improvement rather than law enforcement. Several have argued the permitted speed of 100km/h is too fast for many of our highways. At the same time, they say, it can be too slow for motorways and expressways. They suggest different speed limits for different standards of roads.

They have a point. On highways that twist and turn through hilly terrain it is not safe to have vehicles travelling at 100km/h in opposite directions separated only by a white line.

Many, probably most, drivers are sensible enough to slow when it does not feel safe but that only adds to the dangers when idiots, hellbent on going as fast as they are allowed, try to pass them.

Passing lanes have proliferated on our highways over the past decade or so and no doubt contributed to the lower national toll for a period.

But passing lanes present their own challenges to a line of cars that all want to pass the one that has been slowing them down.

A maximum speed of 80km/h on roads with nothing separating traffic in both directions would seem safer. On motorways and other multi-lane roads with median strips or barriers, a speed of up to 120km/h could be permitted. It is a speed modern cars can comfortably travel when the traffic is light.

Likewise, on urban arterial roads, an increased limit of 60km/h would better reflect the speed that normally prevails anyway.

Shortly before Christmas, according to Police Assistant Commissioner Dave Cliff, the Cabinet approved new guidelines for setting speed limits better suited to each type of road. That should help.