If you think merging into another lane at the last minute is rude and unsafe, you're probably from California. Much of the U.S., outside of the Golden State, is in favor of what's known as the "Zipper Merge" where drivers are encouraged to merge at the last minute when roadways are congested.
Experts say that when lanes become blocked and traffic is heavy due to construction or accidents, the zipper merge is much safer, and keeps traffic flowing better. According to studies done by the Colorado Dept of Transportation, zipper merging can reduce delays by up to 40% in heavily congested areas.
According to the Automobile Association of America, “Study after study shows the ‘zipper merge’ is the safest and most efficient way to merge in work zones and heavy traffic,” says Kurt E. Gray, AAA director of driver training. “To do so, use all lanes fully until you reach the end of the lane, then alternate into the open lane. It does take some cooperation, but it maximizes road space and helps keep things moving.”
On the flip side, they say when drivers merge early, it causes other lanes to slow down (sometimes suddenly), which increases accident risk.
Despite the logic supporting the zipper merge, California doesn't appear to want to get on board. A Caltrans spokesperson told KTLA, “Caltrans is aware of the ‘zipper merge’ but we are not studying or researching it. We are not promoting it to the public.”
Below is a video from the Arizona Departement of Transportation.