Late merging is more efficient

Not everyone realizes it, but the rude drivers are the ones who changes lanes before it's necessary.

From TV news:

Drivers whose lane of traffic is going away might think it’s polite to merge early but when highways are congested, expert advice is to wait and merge late.

Studies show traffic moves more safely and efficiently if drivers use all lanes to a merge point where cars take turns moving forward on the remaining roadway.

But the concept is so counterintuitive to some drivers, HowStuffWorks.com reports a number of state departments of transportation have campaigns to educate drivers about proper merges.

In Missouri, the advice is not to think about merging as one lane moving into the other, but as two lanes merging into one. Colorado tells drivers late merges are a safer, more courteous and more efficient way through cone zones. Kansas created a video of two animated traffic cones discussing how early mergers back up traffic by creating one slower lane versus cars moving ahead with more efficient zipper merges.

The early mergers are a problem because they disrupt traffic by cutting in front of someone not expecting it, they waste a lane of traffic, and then they tend to try to block those who are doing it properly.