<As for a device to measure how much a vehicle weighs as it drives by, I can't think of any way to make it work.

Weigh-in-motion (WIM) detectors are currently being used to monitor highway wear and tear. These are not the methods I started this thread with -- using "bending plate", "piezoelectric", "load cell", "on-board pressure sensors", & "capacitive-based mats" -- but they do weigh traffic in motion.
http://www.ornl.gov/dp121/cbt.htm

A WIM (Weigh-In-Motion) system helps to monitor and keep weights under control so that road wear is minimized. There was a time not so long ago when many trucks were allowed to bypass the weigh stations:
"In 1986 the Port Of Entry (POE) division, operating with the IBM System 36 computer system, reduced the time it took to clear a vehicle in compliance from 2.5 minutes to 41 seconds per truck. However, this same system cannot keep up with the current traffic volume during peak periods. In 1994 the POE was forced to allow 193,000 trucks to bypass the ports without clearance to avoid safety concerns of trucks backing up onto the highway. Another 235,704 vehicles bypassed in FY1995 and 276,236 bypassed in FY1996. These numbers are expected to increase as truck traffic grows. Replacement of the System 36 has reached a critical point for the following reasons:"

My older brother trucks,
Dan