A zipper merge is a traffic pattern that is supposed to have cars merge into one lane, one after another, when they reach a certain point. In theory, it should reduce congestion and minimize delays, but in practice, it usually does anything but.
Game theory, which studies how people make decisions when competing among others, explains why zipper merges don’t work. (Also, keep in mind that the vast majority of drivers on the road were not taught how to navigate zipper merges in driver’s ed.)
If you are the driver in the lane that ends (“Driver A”), your two choices are to either merge at the merger point (correct), which is right before your lane ends, or merge as soon as you can (incorrect), often as soon as you learn that your lane will be ending.
However, in reality, the majority of drivers in the lane that ends will merge as soon as possible. Applying game theory, Driver A is thinking about whether the drivers in the lane that doesn’t end will let them in and will weigh the benefits of their choices.
A cooperative outcome will keep traffic moving and reduce the chances of backups (what the zipper merge is intended to do); however, cooperation can’t be sustained due to individual incentives to defect from it, according to game theory. Thus, Driver A is likely concerned about not being able to merge at the merger point and doesn’t want to get stuck in their lane.
The result is that Driver A believes their optimal strategy is to merge as soon as possible to ensure that they’re in the next lane.
To combat this situation, Departments of Transportation across the country are embarking on campaigns to educate drivers about zipper merges.
I do think these campaigns will be helpful, but ultimately, it’s very difficult to change driver behavior, especially when they’re looking out for themselves.
Illustration by Indiana Department of Transportation