Probably the simplest way of accomplishing this is to set up a trigger volume actor and assign it as a child component along the spline points in your spline’s construction script. You can then set up a map to assign speed limits to indexed points along the spline, and then an interface in the trigger volume to pass the relevant variables on to your vehicles as they pass through them.
First set up some logic in your construction script to add the child components at spline points
Then set up the defaults for the map variable to assign speed limits at specific points along the spline
Next set up some logic in your trigger volume actor to pass the speed limit variable on to the interface when the correct class of object overlaps a volume.
Finally implement the interface in your vehicle and have it set the relevant variables from that interface function.