Is It Always Two Lines?
A double yellow line — you know what I’m talking about. So why do we avoid the topic? It’s come to the point I hesitate to even bring up the subject, especially at parties or family gatherings. And it’s not always simple yellow paint, you know. Many times it’s a product called thermoplastic — a plastic polymer. I know it sounds futuristic but its chemical properties allow it to become soft when heated and hard when cooled. It’s much thicker than paint and lasts a long time. And the best part — it’s expensive.
But what exactly does a double yellow line mean? No one knows, but let me take a stab at it. According to my computer laptop etch-a-sketch thingy, a double yellow line indicates maximum or special restrictions. My traffic engineering instincts suggest if you should happen upon a double yellow line in the wild, turn around and go home.
If, however, you choose to proceed, take note of these specific maximum or special restrictions: a double yellow line (i.e., two solid lines, not dashed) means that particular part of the roadway is for 2-way traffic and crossing the double yellow line for passing is prohibited for traffic traveling in either direction.
And not to overwhelm you with yellow centerline creeds, but there are two more options for two-lane, two-way roadways, 1) a dashed yellow line (where passing with care is permitted), and 2) a double yellow line consisting of one solid line and one dashed line (where passing with care is permitted but only on the side with the dashed line).
Note that a single solid yellow line is not used as a centerline marking on a two-way roadway. And centerline markings on undivided two-way roadways (i.e., with four or more lanes) are always double yellow lines.
Now you may notice some roadways do not even have marked centerlines. The decision to mark a centerline is based on roadway width and the average number of daily vehicles using the roadway. And there are other scenarios where centerline markings are appropriate, but to make those determinations you need something affectionately referred to as, a traffic engineering study, and good luck getting one of those…
Chet Skwarcan (traffic engineer, author, unique insights) with over 25 years of traffic engineering experience — online help available at TrafficEngineering.com/Services