Merging onto a freeway from an on-ramp is different, and zipper merge doesn't usually apply. There's a reason the merge zone is 300-500 metres long (where space permits), and depending on the region, clearly marked with a dashed line. It's up to you to match the speed of traffic, then signal your intention to merge before merging into a gap in traffic. Some on-ramps even have metering lights to space out incoming vehicles to make this process smooth. The only time other vehicles should have to change their speed is in heavy traffic, when a gap might not exist.

> Merging onto a freeway from an on-ramp is different, and zipper merge doesn't usually apply.

I'm not talking about merging onto a freeway from an on-ramp. (I'm told the relevant law for on-ramp merging right-of-way varies by state, in the US.)

> The only time other vehicles should have to change their speed is in heavy traffic, when a gap might not exist.

Heavy traffic is the only time a zipper merge might be relevant in the first place, no? In light traffic, just get in the correct lane whenever you want. It doesn't affect anyone else no matter where you do it.

In fast heavy traffic (when drivers aren't leaving the requisite 4 seconds between cars), there's no reasonable gaps into which to merge. Signaling my intention doesn't change anyone else's behavior, and I don't have the guts to sneak my car into a 20' gap at 60mph. Even if I make it, if the driver in front taps the brakes even slightly, we're all going to have a real bad day.

This is a perfect example of how you can't just transplant one aspect of German driving, in isolation, to American highways. On the Autobahn, nobody's driving full speed for an extended length of time at only 1-1.5 car lengths behind the car in front of them. It's reasonable to say "just merge into the gap" because they leave a gap to merge into.