The accuracy and the precision of Transit Tracker, the Internet- and telephone-based real-time arrival information system of the Tri-County Metropolitan Transportation District of Oregon (TriMet), are analyzed. Although Transit Tracker estimates the arrival times of both bus and rail vehicles, this analysis is limited to bus vehicles. Archived data from TriMet's automatic vehicle location (AVL) and automatic passenger counter systems were used to compare actual bus arrival times with Transit Tracker estimates drawn from logs created in the estimation process. Accuracy was evaluated primarily by calculating the mean of the difference between the actual bus arrival times and the arrival times estimated by Transit Tracker. Precision was determined largely by the degree of variation in the estimates and measured by the standard deviation of the difference between the actual bus arrival times and the arrival times estimated by Transit Tracker. Finally, an arrival estimation error model was developed to test the effects of a series of factors on the degree of estimation error.
