How does Google Maps compute ETA?

Google maps is used to route an individual from point A to point B. It is used by below user groups:

1. Pedestrians- who use the mobile app to find walking directions to a nearby place

2. Drivers: who use in-bulit navigation in the car (or they can choose to use gmaps in their phone)

3. Public transport commuters: who look up for routes using public transport (combination of trains, buses, walking) 

Clarifying questions:

Do we want to focus any particular mode of transportation or user action for computing the ETA?

Interviewer: Feel free to choose any route

Let's focus on user group #3 i.e. public transport commuters. If we focus on cities, this user group will cover the most use cases since it includes daily commuters going for work or other chores, explorers, tourists etc. 

User journey:

1. The user taps on a app (mobile or desktop)

2. Enters the starting address (or uses current location) and the destination address

3. Enters the starting time of the journey (or can select 'now')

4. Selects the preferred mode of transportation 

5. Gmaps displays the entire route and provides the ETA

Route shown by gmaps can be a combination of any of the below components:

train, buses, walking to/from the train/bus station

Google's strength for computation of ETA:

1. Google partners with transportation authorities and data providers to source real-time schedules of public transport options a.k.a buses/trains/ferries

2. Google maintains a huge database of location-based crowd-sourced data updated in real-time. This will give information on any accidents, delays, incidents like elevator out of service at a terminal etc. 

3. Google uses historical data and AI to do predictive analysis on the peak-times and traffic patterns to estimate the speed of the public transport vehicles

4. Google computes (I'm guessing) the average walking speed of a person by tracking the time taken historically by users to walk similar distances (factoring traffic conditions)

5. Google has a vision of all the roads and infrastructure mapped out from satellite imagery, again updated in real-time

So let's say a route consists of 

1. Walk from the starting point to train station (gmap computes the time using strengths #2,4,5)

2. Take train ride (gmap computes time using strength #1,3)

3. Walk from destination station to final destination (gmap computes the time using strengths #2,4,5)

Gmaps will add all these times and factor in the starting time to give an ETA