However, an RMS spokeswoman said they were safer than most other heavy vehicles on the road.

Appin Road has long been a cause for concern, with an RMS study showing 84 crashes and five fatalities had occurred on the section of road between 2008 and 2013.

In the wake of the weekend's news that the speed limit would be lowered from 100km/h to 90km/h, concerns about the RMS approval for larger-than-usual trucks to use the road were raised.

This is despite the RMS study finding that only 11.9 per cent of those crashes involved heavy vehicles, compared to 83.3 per cent featuring cars.

An RMS spokeswoman said it had granted permission for Bulktrans to operate A-double trucks - which are 1.5 metres longer than B-doubles in use on the road - "to carry coal efficiently and productively with minimal impact on other road users in the Illawarra".

The spokeswoman said the trucks met the performance-based standards set by the National Heavy Vehicle Regulator.

"The trucks are more highly equipped than most heavy vehicles on the road with electronic braking system, roll stability support and UltraShift performance automated transmission with manual button gear selection, which means no clutch pedal is needed," she said.

The spokeswoman said RMS had imposed extra conditions on the trucks, including satellite-based tracking technology and regular checks of their on-board scales.