When high-quality information provided by a transit agency isn’t available, Google uses aggregated crowdsourced data to predict public transit crowdedness.
Public transit crowd size
When you search for transit directions, Google predicts public transit crowd size.
Google predicts your vehicle’s crowding by seeking optional feedback from Google Maps users who have opted-in to share their contributions. Google sends notifications to users who have enabled notification and location permissions for Maps on their device, and signalled they are riding a transit vehicle.