That's not how it works. Maps will see that the average speed of cars around Chinatown has slowed to a crawl, and it will route drivers elsewhere. They also get lists of planned closures from local traffic agencies, and you'll see these clearly displayed in black and red with little stop signs. (The quality of this data obviously varies wildly though.)

Accidents on highways are a different story because there are rarely any alternatives, or if there are, they're so much slower that it still makes sense to suffer through the 30 min jam.

I’m guessing GP’s point is that Google also has the locations of all the people not in cars, so Google Maps could take the hint if it wanted. But thus far nobody appears to care enough to fix this. (And apparently in e.g. India cars and pedestrians may routinely use the same roads at the same time, so perhaps such a system might be more fragile globally that our own experiences would lead us to believe.)

Road closures and accidents are crowdsourced things, so ordinary users can contribute intel live from the scene.

Google is able to estimate crowd sizes in a business or on a public transit vehicle. This seems to be based on the number of Android devices reporting their location in a cluster. Maps could obviously make inferences if there were large crowds of people, not moving in vehicles.