"Check Length" sign on highway

[The General]

Bit of a random question but ive been doing a bit of driving up and down the F3 and Pacific Highway and passed this sign a few times and can't work out what its there for.

It starts with a "Start check length" or "Start length check" (can't remember the exact wording) sign and then has a marker every km for 5 km. Anyone have any idea what its used for?

 

[The Colonel]

Checking your odometer......

 

[Twizzle]

its seeing if you speedo distance is properly calibrated

 

[AlwaysGreen]

My length is always the same unfortunately.

 

[***MH***]

I always thought it was a friendly prompt to look down at your crotch.

 

[Apey]

Bob Kelso... 10 inches.

 

[Danish]

Its like a baguette