What is the Defense for an Obstructed Stop Sign

This hypothetical situation involves a traffic ticket in the state of: California.

A motorist has been charged with failure to stop at a stop sign, 22450(a):

Quote:

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Quoting 22450

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(a) The driver of any vehicle approaching a stop sign at the entrance to, or within, an intersection shall stop at a limit line, if marked, otherwise before entering the crosswalk on the near side of the intersection.

If there is no limit line or crosswalk, the driver shall stop at the entrance to the intersecting roadway.

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There was no limit line marking at the intersection and the motorist did not see the stop sign. He cautiously rolled through the intersection, never stopping, and made a right turn. He was subsequently given a citation. Upon further inspection there was in fact a stop sign present but the sign itself was entirely obstructed by a tree. It would have been impossible for any reasonable motorist to see the sign.

What would be the legal basis for the motorist's defense?

This isn't a trick question or anything of the sort, I want to know where in the law it says/implies that the sign must be legible.

What is the Defense for an Obstructed Stop Sign