The Load percentage that appears on predictions is an estimate of how full the vehicle is by comparing how long it’s been since the previous vehicle reached each stop (gap) to the scheduled time between vehicles (headway). These values are averaged for each stop since the beginning of the trip. 100% doesn’t mean the vehicle will be full, but the number of passengers should be close to the amount the vehicle would be expected to have if everything ran on schedule. If the vehicle is normally lightly used or near the beginning of its route, there could be room at 200%. The values will be more accurate as the vehicle approaches the stop.
For vehicles that have not started it's trip, the values are based on the gap between the predicted vehicle arrival times and the scheduled headway. It does not work for MTA subway or Metro North.
It can be disabled in the settings.
TransSee by Darwin O'Connor. Follow on Twitter.