The calculation of ETA on Google Maps begins by identifying the shortest route from the starting point to the destination, which can be based on either distance or travel time and is determined using algorithms such as Dijkstra's or A*. However, the ETA is influenced not only by the shortest route but also significantly by traffic information. Google gathers real-time traffic data from multiple sources, including local transportation agencies, private data providers, user feedback, and location data from smartphones utilizing Google Maps. These real-time insights are integrated with historical traffic data to enhance the accuracy of the ETA. For instance, if a route tends to be congested during rush hours, this will be taken into account in the ETA calculation. Additionally, machine learning techniques are used to forecast future traffic conditions. In summary, the ETA provided by Google Maps is the outcome of a detailed analysis of data combined with various sophisticated algorithms operating in the background.