Should WA and NT drop rural default speed limits to 100kph? Here’s what the evidence says

The spotlight is back on speed limits, as the Western Australian government prepares to host a road safety summit in early September. Ahead of the meeting, at least one expert has called for the WA government to lower default speed limits on rural roads in WA to 100 kilometres per hour, down from 110 kilometres per hour.

While every other state and territory in Australia has a rural road default speed limit of 100 kilometres per hour, the default limit on rural WA and Northern Territory roads is 110 kilometres per hour.

So, would dropping the default limit by just ten kilometres on country roads in WA and NT make a difference? And how might it be enforced? Here’s what the evidence says.

Would it make a difference, and how do we know?

Theory tells us that the faster the speed at which a vehicle crashes, the more likely it will result in serious or fatal injuries. There is a limit to what the human body can withstand.

Now let’s look at what has been found in practice.

Research we conducted at the Centre for Automotive Safety Research showed the risk of serious and fatal injury in all impact types reduced from 4.6% at a travel speed of 110 kilometres per hour to 3.3% at 100 kilometres per hour. That’s a reduction of about 28%.

For head-on impacts, the risk of serious and fatal injury fell from 20.9% to 12.2% – a 41% reduction.

But will reducing speed limits actually reduce the speed at which people travel?

In 2019, a group of researchers reviewed studies that reported on 26 changes in average travel speeds due to changes in speed limits. This included results from three Australian studies.

The review found cutting the speed limit by ten kilometres per hour may be expected to reduce the average actual travel speed by three kilometres per hour.

In other words, even if people didn’t slow down a lot, they still slowed down a bit, which helps reduce risk.

And over time, as drivers get used to the new lower speed limit, they may slow down even more.

But do crashes actually go down?

Yes. Studies from around the world have shown that, generally, when speed limits go down, crashes go down. This is true for both injury crashes and fatal crashes.

The same pattern holds true in Australia, too.

Australian studies looking at the effect of reductions from 110 kilometres per hour to 100 kilometres per hour found injury crashes were reduced by:

Would a speed limit change really help, given how much rural roads vary in quality anyway?

The default rural speed limit applies to all roads outside of towns and cities (except where a different speed limit has been posted).

And remember: just because the default rural limit is lowered to 100 kilometres per hour, it doesn’t mean that will be the limit for all roads. Some roads may still have higher or lower speed limits.

A lower default speed limit on rural roads makes sense because it would make drivers safer on already poor quality rural roads.

Many rural roads are undivided, narrow, have no shoulder, or are unsealed. They may have hazards such as trees close to the edge of the road. Hazards such as livestock, wildlife, heavy vehicles, agricultural vehicles, or mining vehicles are common.

For these reasons, many rural roads are better suited to lower speed limits.

On the other hand, a high quality road – like a freeway or highway with divided traffic flows, sealed shoulders, dual lanes or passing lanes, few or no intersections, and roadside clear ways or barriers – may be better suited to a higher speed limit.