A new road rule that will force New South Wales drivers to slow down to 40 kilometres per hour when passing emergency vehicles "doesn't go far enough", a victim's father and roadside assistance bodies say.

The Slow Down, Move Over (SLOMO) legislation is a 12 month, state-wide trial that will penalise drivers who fail to slow down for emergency vehicles that have pulled over and have their blue or red lights flashing.

Drivers may be slapped with fines up to $448 and three demerit points.

But the father of a young woman who was killed at the scene of a roadside breakdown, Peter Frazer, said calls for the rule forcing drivers to also slow down when passing tow truck drivers and roadside assist had been ignored.

"We're extraordinarily disappointed by the fact the NRMA and tow-truck drivers are not going to be covered. This is what started it and they're not going to be covered," he said.

"It's really quite sad that the NSW Government hasn't seen fit to include them."

Law 'doesn't go far enough'

Mr Frazer has spent the six years since his daughter died in an accident on the side of a NSW highway campaigning for legislative changes to improve road safety.

He started the Safer Australian Roads and Highways (SARAH) group which now drives the National Road Safety Week each year and is frustrated the laws are falling short.

In early 2012, his 23-year-old daughter Sarah was on the road to Wagga Wagga when she pulled into the breakdown lane after her car overheated.

Sarah Frazer's overheated car was being loaded onto a tow truck by Geoffrey Clark when Kaine Barnett's truck hit them, killing them instantly. ( ABC News )

Moments after her car was loaded onto a roadside assist tow truck, a courier truck drove around a bend at speed and smashed into it.

Ms Frazer and the tow truck driver sent by NRMA, Geoffrey Clark, were killed instantly.

"The legislation that's been put in place by the NSW Government would not have saved her life then, and it would not have saved the life of the tow truck driver," Mr Frazer said.

NRMA spokesperson Peter Khoury has agreed, saying the safety of not just roadside assistance workers but other first responders is at risk.

"The whole purpose of this legislation is to provide protection to people in dangerous locations," he said.

"This legislation only does half the job."

Peter Frazer is demanding the NSW Government change the law to include tow trucks and roadside assistance vehicles. ( ABC News: Jesse Dorsett )

Mr Frazer and the NRMA want the NSW laws to be in line with those in brought into Western Australia earlier this year which require drivers to slow for police, fire, ambulance, RMS and NRMA-equivalent workers, as well as tow trucks.

New rule 'impractical' for truck drivers

The new laws are being labelled dangerous and impractical by the National Road Transport Association, which represents the trucking industry nation-wide.

Richard Calver, a spokesperson from the association, said member feedback from similar laws instated in Victoria identified dangers in having to reduce to 40kph on busy roads and highways.

"The [NSW] rule's applied in a manner that's reasonably practicable because it's not always appropriate to slow to 40kph when around light vehicles," he said.

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The association has also identified roads with hills and corners as an issue in obstructing a clear view of incidents.

"We understand the purpose behind the rule. It just has to be modified [to be practical] so if a heavy vehicle doesn't slow in time it doesn't get that $448 fine and three demerit points," Mr Calver said.

One year trial to reveal any changes

The NSW rule will be trialled for 12 months from September 1 before it is reviewed in September 2019.

"We want to make sure that we monitor and evaluate how the rule applies and see if it has any other consequences associated with congestion or other issues," said the executive director of NSW Centres for Road Safety, Bernard Carlon.

"Then we'll have the opportunity to review how that's all gone and whether it should remain as a permanent road rule, or whether there should be other vehicles considered as well."

This will include the possibility of including vehicles with orange flashing lights such as tow trucks and roadside assistance, as well as garbage trucks, private security, escort vehicles, over-sized vehicles and special purpose vehicles, he said.

Mr Frazer says he had waited for change long enough.