system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does Google Maps calculate my route so fast? What sort of logic do the algorithms use to find the fastest route?

Additionally, how does it calculate the ETA with the current traffic conditions?
gpt
Google knows the speed limit of most roads, you can add them yourself on Google Map Maker. They also know where many stop lights and stop signs are.  
  
Since they know these things, they know a route which has a 10 mile road with a 60mph speed limit is faster than an 8 mile road with a 45mph speed limit. They also consider a route with the least amount of stop lights/signs is better.  
  
As for traffic, every phone that uses Google Maps for navigation has location services turned on, meaning they know how many cars are on the road, and how fast they are moving. Apple Maps does this also, but because Google Maps is cross platform, their traffic data is more precise and abundant.  
  
Google takes all of these factors in determine ETA.