A drive-over tire tread depth measurement system is configured with components for preventing or impeding environmental contaminates from falling through openings in a vehicle support surface or cover plate onto sensor optical windows and adjacent surfaces. The components may include active mechanisms, in the form of air discharge assemblies, movable shields, guards, wipers or deflectors, and/or passive components such as drip edges, water-accumulating textured surfaces, flow diverters, and selectively placed flanges or fixed guards which operate cooperatively to displace accumulated contaminates, debris, or liquids from the optical windows and adjacent sensor surfaces on a periodic basis.