F8 in TrackEd, it's pretty straightforward actually.
Shift+click to place the cursor somewhere on the track.
Ctrl+a will put a waypoint that is 'forward' from the one currently selected.
Ctrl+s will let you select a waypoint (by number; they're all clearly displayed)
Other options have buttons, but those 2 are the main things you'd use.
Basically you set the cursor where you want the traffic to start, then click somewhere forward, ctrl+a, repeat until it gets back near the start. Then choose "set forward waypoint" and input the number of the first waypoint on the loop to complete it.
As far as I can tell, the waypoint speeds pretty much update the traffic's target velocity when they cross the waypoint. So if you want a car to slow down you need to set it up with enough distance to get to the target speed.
Adding leftward/rightward waypoints gives the traffic random choices when it hits those points, and TrackEd displays them with L/F/R when you have the wp selected.
One problem I've had is that the traffic tends to automatically acquire waypoints when it drives over them, so the paths need to be laid out to avoid hitting waypoints on other routes. It might be good to have some sort of 'detection radius' you could set on points so this could be more obvious and avoided.
Anyway, that's why I have 3 points per turn - more and they just overlap too much. It does make the AI steer a bit suddenly.
Most of the time they do avoid collisions, but they can only see what's in front of them, so when two come at each other from the side they don't always have time to stop.
If you need to remove waypoints, select the first one along the route you want removed, cause it automatically jumps to the 'forward' waypoint when you edit waypoints, and that way you can just repeatedly hit remove.