The track speed is also controlled by line environment factors such as caution signals, junction settings, and the slowest portion of that section of track.
A line with a red signal about 3 miles away, will cause yellows ahead of the AI driver. They will automatically drop to 1/2 speed, or 23 mph max when they pass through a caution signal.
A junction set wrong for them, and they will slow down for that even if it is many miles away. This will cause intermediate signals to turn yellow, and slow them down.
If there is a slow section, they AI will slow down over the whole section they're driving on to prepare for the slowest section.
John