The new rule, enforced from today, requires all drivers to slow down to 40km/h when they pass stopped emergency vehicles flashing their blue and red lights, regardless of the speed limit.

Failing to do so could result in a $448 fine and the loss of three demerit points.

Steve Pearce, Chairman of the Motorcycle Council of NSW, says the rule could prove deadly on motorways and freeways, where motorists are generally doing 110km/h and might have to slam on the brakes to be compliant.

"By suddenly reducing the speed limit on a major carriageway to 40km/h we are effectively swapping the safety of emergency workers for the safety of ordinary motorists," said Mr Pearce.

"Anyone who has driven on a high speed road knows the effect of braking suddenly from 110km/h to 40km/h.

"The resulting snaking of traffic can produce a trail of rear end collisions, and more often than not this is what happens."

Mr Pearce said that Motorcycle Council members have voiced their concern for the new rule "almost unanimously", as they consider motorcyclists to be "the most vulnerable of road users".

"The effect of a rear end collision on a motorcyclist will most likely be a serious injury or fatality. So we are very concerned about this," he said.

Richard Calver, Adviser on Compliance for the National Road Transport Association, says the organisation fears mostly for truck drivers, who have a harder time slowing down than light vehicles.

He says that if truck drivers slow down to the extent of their "practicability" but don't match the 40km/h mark, it's unfair for police to slap them with a penalty.

"We're saying if you're coming around a corner and you only see the emergency vehicle at that point in time, then it might not be practicable for a heavy vehicle to decelerate to 40km/h as it passes," said Mr Calver.

"But as long as they're decelerating, then they've done the right thing."

Mr Calver said that while NatRoad supports the state government's intention to trial the rule for 12 months before implementing it next year, they'd like to see it modified before it's a permanent law.

"We absolutely support a trial. As to whether or not this increases safety generally on the roads, you've got quite a few phenomenon here [to consider]," he said.

"There's no magic in 40km/h. [For the same road rule] it's 25km/h in South Australia, for example."

The South Australian law was introduced in 2014, and Victoria introduced a 40km/h limit in July last year, although that state's motorists only face on-the-spot fines of $277 if they break it, and don't lose demerit points.

Road Freight NSW, the state's peak industry organisation representing trucking operators, agrees that the new road rule should factor in the differences between light and heavy vehicles if it is to be "equitable".

"It would be a perverse outcome if a heavy vehicle was involved in an accident as a result of light vehicles slowing down quickly to 40km/h when the heavily vehicle cannot," said CEO Simon O'Hara.

"Our members will adhere to all NSW laws, but would seek to ensure that these laws are equitably applied in circumstances where one size doesn't fit all."

The decision to implement the new rule in NSW was based on input and advice from a working group involving Transport for NSW, Road and Maritime Services, NSW Police, NSW Rural Fire Service, Fire & Rescue NSW, NSW State Emergency Service, Ambulance Service of NSW, the Volunteer Rescue Association, the NRMA and industry representatives.

It also requires that motorists give way to any person on foot in the vicinity of a parked emergency vehicle flashing its lights.

It does not apply to drivers who are on the opposite side of the road of a parked emergency vehicle when the road is separated by a median strip.

Bernard Carlon, executive director of the NSW Centre for Road Safety, said drivers should "always start slowing down in a safe and controlled manner as soon as they first see blue or red flashing lights, taking into account the current road conditions including surrounding vehicles".

"We want to make sure emergency service workers protecting us on our road network don’t become casualties while doing their jobs," he said.

with Jenny Noyes