Gtfs vehicle position vs actual position requirements, expectations and testing

[glen.m...@gmail.com]

Any experience or studies on what degree of update speed and accuracy between the position of the vehicle on the road and the location as posted on the gtfs server need stop be?
Update frequency?
Update time from unexpected event to update on server?

If the location as served is too old, then the position is too far off to be useful. Any experience as to what number that is? 45sec? 2 min?

Also anyone know of a method to test the accuracy of the gtfs output vs real location in the field. Vehicles are tracked at a central system.
Thanks