The signs, which feature the message WA Naturally Thanks You, were installed as part of the Regional Roadside Litter Prevention Project.
The $185,000 project is supported by Keep Australia Beautiful Council (WA) and Main Roads WA and it will continue until the end of the year.
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Mundaring Shire President David Lavell said the project focused on an issue important to the Shire and encouraged people to dispose of their rubbish in the correct manner by promoting a sense of personal responsibility.
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READ NOWCr Lavell said he was confident thanking people in advance would help improve long-term littering behaviour to keep the environment clean.
The innovative project includes complimentary litterbags at some roadhouses in the local area.