system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5 How does a car that turns off in a stop beneficial?

Today I was outside of a store. Infront of me a slow traffic and a BMW stops and you can hear the engine turn off and on. 

What is the benefit of a car doing that? Wouldn’t that use more mechanic and energy when it constantly turning on and off?
gpt
It is a required feature on European models, the reasoning isn't that you will save a bunch of gas but that it helps with air quality in urban areas. Emissions from idling vehicles are the worst kind of pollution, the kind that isn't the result of any useful work being done.