Road Rules: How speed limits are decided and set


Question: I live in a rural town and my neighbors have been harassing us lately to slow down on a road that by law is 25 mph. They have today now posted a “slow down: speed limit 10 mph” sign they bought off the internet. Does this sign have any legal standing?

Answer: Can you imagine if you really could just decide what you wanted the speed limit to be in front of your house and then put up a sign? What happens if I think the speed on my street should be 20 mph, but my neighbor prefers 50 mph?

With enough opinions, we could be changing speed limits at each property line, and maybe even have different speed limits on each side of the street. Of course, this is all nonsense, as it’s not up to the individual residents on a street to decide the speed limit.

Instead of your neighbor picking a speed limit, the maximum speed for a given type of road is defined in the Revised Code of Washington. The section of the law titled, “Basic rule and maximum limits” sets speed limits at the following:

• City and town streets – 25 mph

• County roads – 50 mph

• State highways – 60 mph

If you’ve paid attention to more than a handful of speed limit signs, you’re probably saying, “Hey, wait a second, there are a lot of roads that don’t match those speed limits. What gives?” I’ll tell you.

The next two sections of the law allow for both increases and decreases in speed limits. However, that doesn’t mean that a neighbor, or any other person, can just decide to change the speed limit. The law states that the decision must be determined “upon the basis of an engineering and traffic investigation ...” There’s a bit more to the law, but the point is that setting a speed limit is supposed to be rooted in data that supports a safe and appropriate speed for a given road.

Your neighbor’s sign has no authority behind it, but there’s more. Not only is it unenforceable; it’s also illegal.

Unsurprisingly, you can’t just put up official-looking traffic signs on the roadway. Here’s what the law says: “No person shall place, maintain or display upon or in view of any highway any unauthorized sign, signal, marking or device which purports to be or is an imitation of or resembles an official traffic-control device ... or which attempts to direct the movement of traffic ...”

Out of curiosity, I went online to a site that sells signs like you described to see if I could tell from the reviews how they were being used. To most purchasers’ credit, they were putting them on long driveways and private roads.

But there were also folks who wrote, “My across-the-street neighbor and I were tired of cars speeding past our homes, so now we each have a sign in front of our house.” And, “Purchased for myself and my neighbors. They have helped to slow the traffic down in our street.” Another person who put up a sign wrote, “Now how to enforce the speed limit?”

I can easily answer that question: You can’t. No officer is going to enforce an arbitrarily chosen speed limit sign on your behalf. 

Citizens can be a part of changing a speed limit, by requesting a review from their local government. If a speed study is done, it may or may not result in a speed limit change.

And if it does, I’m confident it won’t be 10 mph.

Doug Dahl is a manager with the Washington Traffic Safety Commission, Region 11 and publishes


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