I don’t know about the scales along the freeway for 18 wheelers, but when we used to take a flatbed load to a metal scrap yard, they’d weigh first the front wheels, then the back. So did they simply add the two together to get the total weight of the truck?
Seems simple enough, and I’ve always assumed they did, but I’m wondering.
Peace,
mangeorge
Yes, the weights can be obtained separately and added together. Many scales for highway weigh stations are not long enough for the 80 ft+ length trucks often have and therefore they weigh the axles in groups or even individually. Truckers can sometimes lock their breaks to move the weight around but the weigh station operators can watch for this. If they want they can also make a driver stop at each axle and wait until their load settles out.
On highways, dynamic in motion weight sensors always weigh axles individually and then add them together.
Additionally, weight operators are often interested in a truck axle group being overweight. The overall weight may be legal, but if the back two axles are overweight the truck driver still gets a ticket.
Thanks. I just neded reassurance. 
A factiod, for those interested.
I worked for a time at a research lab. Pavements were one of the things involved.
You can design and build a road for load XYZ.
If every single load is say 80 percent of that spec, millions and millions of “passes” can occur before the pavement eventually breaks/cracks. Get it closer, like say 90 percent, and its more like thousands. Closer still and its a few handfuls. And one single load over spec will crack it. These number are just off the top of my head, but the point is that failure rate vs load is extremely non-linear.
ONCE the pavement cracks, the pavement goes to shit fast. Now, loads MUCH smaller than spec will contribute to its continual degredation, where as before, the smaller loads were basically only wearing away the top surface (very slowly). And now loads like say 80 percent or more REALLY help the degredation along, whereas before they were doing it very slowly.
So, all it takes is one seriously overwieght truck, or a handful somewhat close to the spec, to mess up a road. This can take YEARS offs a roads life, costing big money and aggravation for everyone during reconstruction.
Truckers like to grip about all the fuel taxes they pay. But they are the ones doing pretty much all the road damage even when they are legally loaded. And the overwieght ones are really screwing everyone (even themselves) over.
Engineering rant over.