Mobile cameras will be deployed in Leith next month to catch 20mph speeders . Picture: Safety Cameras Scotland

The comment came as it was revealed that speed camera vans will be deployed to enforce Edinburgh's 20mph zones for the first time next month.

Chief Superintendent Stewart Carle, the force's head of road policing, was giving evidence to the Scottish Parliament's rural economy and connectivity committee on a member's bill for a default 20mph limit on most 30mph roads in Scotland with street lighting.

Mr Carle said: "I would have concerns that the bill seeks to impose 20mph as a blanket

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"20mph will not be a priority.

"The majority of casualties are on higher-speed roads.

"I must prioritise where there is the biggest impact.

"Suddenly switching a lot of resources from faster roads would not give the same gain."

"Mobile safety camera units are not calibrated for 20mph."

Mr Carle also said mobile speed camera vans could not currently detect drivers breaking 20mph limits.

However, East Safety Camera Unit manager Andy Jones told The Scotsman this was incorrect and they could detect speeds of 20-30mph.

Mr Jones said a van would be deployed in Ocean Drive in Leith from the first week of April for three months to enforce the 20mph limit.

Further deployments could follow a review.

He said: "To date, the mobile units haven't been deployed in Edinburgh 20 zones.

"However, we will be on Ocean Drive, Leith, next month, and may identify new sites during the review."

A researcher evaluating the 20mph zones which now cover 80 per cent of Edinburgh's roads told the committee that speeds across the capital had been reduced by 1.6mph.

Dr Ruth Jepson, a reader in public health at the University of Edinburgh, said among motorists who had previously driven at 24mph or more, the reduction was 2.3mph.

She also said that while 25 per cent of drivers had opposed the lower limit before it was introduced, this had now fallen to 20 per cent.

Dr Jepson said they had found "it's not as bad as people think it's going to be."

Green MSP Mark Ruskell's member's bill is to change the law to reduce the speed limit in residential streets and built-up areas.

However, transport secretary Michael Matheson said there are a number of "challenges" with the Restricted Roads (20mph Speed Limit) (Scotland) Bill.

He said: "We don't know the numbers of restricted roads in Scotland, there are some restricted roads actually you wouldn't want to have as 20mph zones.

"There are roads which are not restricted you would possibly want to have as 20mph roads as well.

"This is a bill that is intended to apply not to a town or a city, but to a country.

"And we are in a situation where our local authorities don't have the information around restricted roads.

"There are thousands of restricted roads in Scotland, but because most of it was done on paperwork over many, many decades, it would be a massive undertaking for local authorities to go through in order to collate all that information and identify that information."

The minister also said it was not known how much it would cost to introduce the legislation - suggesting the indicated financial impact on councils of £21 million to £22 million could be an underestimate.

He was also clear there is no funding in his budget to meet the cost of the change.

Mr Matheson told the committee: "Any financial support we would have to give to local authorities - and I recognise we would have to give them financial support to assist them with this matter - would have to come out of existing budget allocations."

Speaking after the meeting, Mr Ruskell said: "The evidence is clear: introducing a 20mph limit in residential areas across Scotland would save lives.