Because a longer traffic jam propagates through the entire road network, possibly blocking earlier exits, making the entire network less efficient.

I am German and can confirm that the zipper rule is holy here, but people often have problems following it, especially unexperienced drivers afraid of merging. Very often, there are additional signs to remind drivers of the rule [1][2][3].

[1] https://www.schilder-versand.com/assets/gfx/product/300x300/...

[2] https://upload.wikimedia.org/wikipedia/commons/thumb/e/eb/Hi...

[3] https://www.schilder-versand.com/assets/gfx/product/300x300/...

It can help move traffic up to unblock exits further behind, which reduces congestion for those not actually travelling through the bottleneck.

Once example I see often that is more length sensitive is dual left turn onto a highway onramp that merges to one lane before entering the highway.

If people use both lanes equally all the way to the merge point you can fit more cars through each turn light cycle and then the merging process continues even when the light has changed.

Often people merge early and leave room in one lane, which leaves some cars stuck on the other side of the traffic light who could have made it through.

The problem with early merges is that there's no way people that merge in are at highway speeds when they merge, causing everyone behind them to have to slow down, which eventually can cause traffic jams. Just use the whole lane, make SURE you're going the same speed as the rest of traffic. The confusion there is that because of an existing jam or slowdown, people are already at a matching speed at the start of the on ramp. Or so they think.

>The problem with early merges is that there's no way people that merge in are at highway speeds

If you drive anything reasonably modern and not grossly overloaded/under-powered you can plant your right foot and be going traffic speed (or close to it) by the end of all but the smallest on-ramps. Most traffic is not made up of box trucks and panel vans so there is no technical limitation that prevents zipper merging on most on-ramps most of the time. Going close to traffic speed makes it a lot easier to merge but a lot of people aren't comfortable doing it that way.

I didn't read this as applying to 'on ramps' (we call them sliproads in the UK) that are lanes merging onto a highway/freeway/motorway. It wouldn't make sense under any circumstances for people on the highway to slow down and stop to let traffic join! I think this is referring to two 'equal' lanes of one road that become one — and, thus, merge. I don't know if they're unusual in other countries, but they're definitely a thing in the UK.

The length of that q is halved, imho traffic issues are caused by one bottlenecks tailback into a junction or similar and creating a bottleneck elsewhere.

its like bandwidth. you are keeping twice the bandwidth up until the last possible moment, meaning the bottle neck is going to be "fed" as efficiently as possible with half the chance of causing a knock on effect elsewhere.

but this is not just a german thing.. in the UK there is a curved arrow sign on the road where one lane ends meaning "merge in turn"

you will also see signs saying: "use both lanes" and "merge in turn" (but as text) which are all the same thing.

you will often see it say after a junction or roundabout where maybe 200m later the road goes from two to one lanes. encouraging you not to just queue right through the previous junction and block that up too.

If you have 200 meters of road to zip, you have more possibilities to merge and can find the best spot to merge into. Also, multiple cars can merge in parallel.