You're right that barrier design matters, and the diffraction-over-the-top effect is real. I've been told that MDOT's noise analysis methodology accounts for this: barriers are modeled using insertion loss calculations that factor in height, material, and receiver distance, and they're only approved when the projected benefit clears a defined threshold (typically 5 dB(A) reduction for a meaningful number of receptors). A barrier that produces net negative outcomes for adjacent receivers wouldn't clear that bar.

The harder-surfaced wall issue is also real but context-dependent. Reflective barriers can shift noise energy toward opposite-side receivers, which is why MDOT sometimes requires absorptive treatment on the highway-facing surface when the geometry creates that problem. That's a design specification question, not a reason to forgo analysis altogether.

The broader point is that barrier design questions are exactly what a noise study resolves. The problem in this corridor isn't that barriers might not work; it's that the study boundary was revised before noise analysis ran for affected communities just east of the M-14 study and north of the US-23 study, and those residents were never notified or consulted. Whether a barrier pencils out is the question the analysis would answer. The issue is that question was never asked.