Okay so this is a little old but I didn't see it anywhere here which was surprising.
99PI (99 Percent Invisible) is the best podcast ever, and it focuses on all aspects of design.
He had a bit or two about the W4-2 sign. I'll link to the 2nd article which links to the first, but thought you all would enjoy
https://99percentinvisible.org/article/lane-ends-merge-right-10-redesign-ideas-w4-2-road-sign/ (https://99percentinvisible.org/article/lane-ends-merge-right-10-redesign-ideas-w4-2-road-sign/)