Qld police defend speed margins

Police are staying tight-lipped about how far they've slashed tolerance margins for speeding on Queensland's roads.

Recent media reports say there has been a 47 per cent hike in the number of motorists caught driving up to 13km/h beyond the limit - the lowest level speeding offence - since new rules were introduced in July 2013.

The level at which motorists can speed over the limit without being fined by speed cameras has been adjusted three times since then, with cuts in June and September 2013, and once more in the second half of 2014.

Acting Assistant Commissioner Mike Keating was pressed to reveal what the current levels were but he said no policing jurisdiction in the world publishes tolerance levels.

But he was forced to concede New Zealand had, at least temporarily, been an exception to that rule.

"We're not going to publicise the tolerance level because that would create a de facto speed limit," he told a media conference in Brisbane.

While it was too early to draw a direct link, he said road statistics coinciding with the program's introduction had shown positive signs.

"In 2014 we had the best year ever for road crash fatalities in this state," he said.

But when he was asked to explain if there was any concrete data linking lower tolerance levels with fewer crashes and fatalities on the roads, Mr Keating said he couldn't point to specific studies.

"There's certainly extensive research about the risk of speed ... I think it's reasonable to draw a correlation," he said.

He said it could take between three and 10 years before the revised policy's effect would show accurately in traffic data.

Debate about this week's reports about the margins was a good thing because it got people talking about safe speed limits, he said.

Queensland has 22 fixed cameras.