You might think it's the most basic of road rules.

The left lane is for passing - the right lanes are for normal driving.

CDOT decided some people needed a reminder - so they're making the rule clear on overhead signs on I-70.

Hundreds of people are weighing in on social media - many thanking CDOT and complaining that the rule isn't forced more often.

Trooper Josh Lewis with the Colorado State patrol says, “we receive a lot of complaints and a lot of people seeing this as an issue.”

For that reason CDOT wanted to remind people that interstate lanes aren't created equal.

“A lot of times we talk to our customers about how to drive well in Colorado,” said Amy Ford, spokesperson for CDOT.

They aren’t just reminding people about driving well they have to also make sure they know to drive legally.

“In the state of Colorado on a highway if the speed limit is 65 or more and there are 2 or more lanes the left lane is reserved as a passing only lane,” said trooper Lewis.

CSP says not everyone follows that rule and they have last year's receipts to prove it.

“The Colorado state patrol made over 4000 stops for left lane violations,” said trooper Lewis.

He says incorrect left lane driving can cause a domino effect that ends in something serious.

“It's the people who get that road rage who follow too closely, who speed, they change lanes quickly these are the types of violations that cause crashes,” said trooper Lewis.

Some drivers showed their admiration for CDOT’s efforts on social media.

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Ford said, “We had someone who noticed the sign in our public education campaign and was excited and said finally!”

Others, questioned whether it's okay to speed when you need to pass.

“There is nothing that gives you an exception to the speed limit law so even if you are making an active pass you're not allowed to break another law in order to do so,” trooper Lewis said.

Troopers say if you flat out just didn't know the rules, it's time to learn.

“Ignorance of the law is no excuse it has been a law for years,” added trooper Lewis.

Again, the left lane is for passing only, when the speed limit is at least 65 miles an hour, and there are 2 or more lanes.

CSP did say enforcing this can be tough.

They have to see a driver violating the rule multiple times or doing it for several miles, before pulling them over.