Via fence distance to the trace?
Creating an answer from the comments...
H and W are the two primary contributors to the trace impedance. The traces as you've shown couple much more strongly to the reference planes than they do to the via fence, which is edge-on to the trace. So the distance to the via fence has a small effect on the trace impedance, compared to H and W. Not zero, but small.
The reference plane (doesn't have to be a GND plane) contributes the most to the impedance because it is broadside to the trace. If the via fence is closer to the trace than H, at some point it will start to have an appreciable effect on the trace impedance. But the trace couples rather weakly to the via fence, and also to another trace (like in a differential pair), at least for traces made with 0.5 oz copper, ~0.7 mils thick (T in your diagram). And as for S, it depends. If S is greater than ~2H, then it has very little effect on the impedance.
One more thing. Via fences also provide isolation between signals. In some applications this is the primary reason for using via fences.