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Explained: How heat affects roads, trains and planes

Related Story: A freeway melted in Victoria — and it's only going to get hotter

Roads can melt, train tracks do expand and, when temperatures are high enough, the book on how to fly some planes literally hasn't been written.

This is how heat can affect the three modes of transport we depend on most.

Roads

They could melt.

The materials that make up our roads are heated when they're laid and rolled into place.

So, when temperatures get high enough and traffic continues to wear on the surface, the bitumen in the roads can "reactivate".

That happened on the Hume Freeway recently.

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A spokeswoman for VicRoads said hot weather could result in "bleeding of the road surface, which occurs when the bitumen becomes reactivated by warm temperatures and becomes soft and sticky".

"This, combined with higher than usual traffic volumes, has affected the pavement on the southbound carriageway of the Hume Freeway between Tallarook and Broadford," VicRoads said.

The temperature in the area hit just over 30 degrees Celsius at 5:00pm on Friday afternoon.

Of course, not all roads will wear the same.

The grade of bitumen and traffic pressures the road is under are just two of the factors that determine how a road wears during extreme temperatures.

Rail

Trains are likely to go slower.

It's common practice for rail operators to slow trains down during periods of high temperatures because tracks can "expand" or "distort".

In Melbourne, Metro Trains has published information on how the heat affects its system.

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"When the track temperature of a line reaches 55 degrees or higher, the maximum speed limit is restricted to 80 kilometres per hour," it reads.

"The tracks can expand in extreme temperatures and trains must travel at slower speeds to ensure customer safety."

When the ambient temperature is forecast to reach 42C, the maximum speed is reduced to 70kph, according to Metro Trains. Its trains usually run at 110kph.

South Australia has been experiencing 40C temperatures this weekend.

Adelaide Metro has a plan that defines "extreme heat" as "two or more consecutive days with forecast 40C plus temperatures based on the Bureau of Meteorology website".

That plan to reduce services mostly affects weekdays, but "trains may also operate at reduced speeds as a safety precaution," according to information published online.

Sydney Trains will also limit speeds during extreme temperatures, as will TransPerth.

"Heat speed restrictions have been imposed every summer in Perth for more than 30 years," information published by TransPerth reads.

"However, the impact has been greatly reduced as the Public Transport Authority has progressively replaced wooden sleepers with concrete.

"Track with concrete sleepers is much less affected by the heat."

Air

Planes can be delayed or grounded.

In June, several major US airlines cancelled and delayed flights out of Las Vegas and Arizona as temperatures soared to the high 40s.

American Airlines cancelled flights out of Phoenix after temperatures reached 49C.

Aviation experts said hotter air was also thinner, causing a decline in performance for jet engines, especially during take-offs.

Also, there might not be any instructions on how to fly the aircraft when the temperatures get too high.

The Conversation looked at the how extreme heat affects planes.

"The July heat-related Phoenix flight cancellations happened at least in part because airlines' operational manuals didn't include information for temperatures above 118 degrees Fahrenheit (47.8C) — because that kind of heat is historically uncommon," authors Ethan Coffel and Radley Horton reported.

"It's another example of how procedures may need to be updated to adapt to a warmer climate."

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