Opinion: Same road, different speed limit?

Target Zero Manager Doug Dahl addresses a question about speed limit signs going into and leaving town

Doug Dahl
Target Zero

Q: I am confused as to why the speed limit signs going into and leaving town are often not across from each other, meaning, I guess, that going one direction it is okay to go 50 mph, while in the other direction the limit is only 35.

A: I’m confused too. Especially since you use “often” in your description. Now that you’ve brought it to my attention, I’ve been paying attention to speed limit transitions, and the ones I’ve come across have all lined up, more or less. But that doesn’t mean they’re not out there, somewhere. As the alien hunters say, absence of proof is not proof of absence.

Assuming they are out there, (the roads with two different speed limits in each direction, not the aliens) the question I have is, should they be? Traffic laws in the Revised Code of Washington don’t have anything to say about differing speed limits in opposite directions, but I didn’t expect they would. Those laws are for road users, not road designers.

The guidelines for designers come from multiple sources, including the Manual on Uniform Traffic Control Devices, the Washington State Department of Transportation (WSDOT) Traffic Manual, local city or county codes, and other engineering reference books I’ve seen on the shelves of traffic engineers. I haven’t found a manual or code that outright states that different speed limits in opposite directions are either permitted or prohibited, but the manuals I have read assume that speed changes will line up for both directions of travel. For example, the WSDOT Traffic Manual directs the folks designing and building state highways to “locate a sign for each direction of travel, opposite one another at the speed zone boundary.”

Earlier I mentioned that the signs I saw lined up, more or less. Maybe it’s the “more or less” part that you’re observing. Sometimes it might not be possible to perfectly align the speed limit signs. The Traffic Manual allows for some offset “if existing features prohibit opposite installation.” If the road geography or other required signage makes it impractical to put the speed limit signs exactly opposite each other, there’s some wiggle room. That offset can be up to 150 feet in either direction from the speed zone boundary. Conceivably then, if the speed limit signs on both sides of the road need to be offset in opposite directions, the signs could be up to 300 feet apart.

The manual continues, “If the signs cannot be installed within these parameters, the speed zone boundary may be changed by the State Traffic Engineer to accommodate sign installation.” From that I gather that the state is not a fan of different speed limits in opposite directions. They’d rather move the speed zone boundary than have disparate speed limits for more than 300 feet.

I realize I made a big assumption in answering your question – that you’re referring to speed limits in Washington State. As I was researching, I did come across what the province of Alberta calls “differential speed zones.” Just to keep things confusing, in the US similarly named differential speed limits are when two different speed limits are posted for different vehicle types, like one speed for passenger vehicles and a lower speed for trucks. In Alberta, differential speed zones are just what we’ve been talking about, and they’re allowed (in limited instances) in that province, confirming that they’re out there, somewhere. Again, that’s differing speed limits, not the aliens.

Finally, I’d be remiss not to close with this: Wherever the speed zone boundary is, the import part is actually following the speed limit.

TheWiseDrive is hosted by Doug Dahl, a Target Zero manager for the Washington Traffic Safety Commission.