Yes, the AI will begin to fail over time and are more prone to do this when there are lots of them. On my very large custom route, a route I started in January 2004, I have about 30 active drivers, dozens of static consists, multiple portals sending out many drivers in addition to the 30 captive ones, and lots of destinations covering close to 189 route miles. With a route this large, things begin to breakdown. I consulted with N3V on this issue and they said yes, the AI subsystem needs to be rebuilt, but that is extremely complex a job and they do not have the resources to dedicate to the task at this time. The other issue they explained is fixing the problem as it stands will break things more and that's something that isn't good. As time as gone on, they have fixed a few things but not at the extent that they really need to do.
Creating a session is really programming using OOP, or Object-Oriented Programming. Instead of writing subroutines of code, this is already done for you and by you placing the driver-commands and other rules in your session, you are actually putting these subroutines and submodules into a program to operate the session.
When first creating your driver schedules, use KISS. Peter mentioned this before as well. Once you've got the AI doing things simply, you can then add in triggers, popups, and other fancy stuff.
What you need to do is recreate your schedule from scratch one AI driver at a time and test each and every route as each driver is added. This will have the cumulative effect of each driver interacting with each other. By setting up each driver one at a time like this, you can doublecheck your schedules to ensure that they are working properly. I really recommend setting up the schedules first using the Schedule Library first because that will save tons of time later.
In addition to signals and track marks, you need to pick and choose your driver commands carefully based on the situation. Do you need to use Drive instead of Navigate to or via? The situation depends on which ones to use. The Drive commands are more controlled and direct whereas the Navigate commands allow the AI to seek their own path and the not-so-direct route to a destination. AI will also be more apt to turnaround using the Navigate commands because of this.
What Peter said regarding employees is true here as well as elsewhere. In the US many companies offer a 401K retirement plan, which comes with matching contributions, a matching split to FICA for Social Security and Medicare, and health insurance. This is well and above the weekly compensation, vacation, and sick pay. When added up, this can mean a small salary of around $20K a year costing a company close to $100K a year with compensation benefits.
