Walking the roads is not practical, only 1.5-2 miles of road per man-hour can be inspected that way because in most cases the inspector would need to back track down the road to view the opposite side lanes.
A potential solution--inspectors on bikes. Bikes are sufficiently fast enough to inspect 5-6 miles per man-hour while getting a very direct view of the road surface. They can easily be configured to carry whatever measurement equipment might be needed to analyze a given surface, even lighter vehicle mounted equipment could be attached to a trike if a more stable platform is needed. Also, a bike or trikes sensitivity to road surfaces won't mask rough roads from the operator like a motor vehicles suspension would.
Isolating the inspectors from the roads may be part of the problem with our deteriorating roads. Let's get the inspectors closer to the inspected--On Yer Bike!