Re: Split Plane
Yes, you are correct. Critical signals would be routed on the adjacent signal layer such that they passed over the "bridge". DC or low edge-rate signals can safely go over a split, but not fast risetime signals.
To pass over a moat, or a split, with the least amount of signal integrity impact, one would supply return path vias connecting either direct traces or other contiuous planes. The return signal would be forced to another layer, but the loop size would be minimized.
The ideal solution, of course, would be to use continuous reference planes for all signals. This is not practical in the real world. Even solid planes that we call "continuous" are punched through like Swiss Cheese with pads, vias, cutouts, etc. The use of multiple planes also inceased the board thickness, weight, and cost. You have to compromise, and one of those compromises is to use split planes.