the best way I've seen it described is it "represents the effects of the vehicle on a bridge while crossing it. The effect is a combination of gross weight, weight distribution of the vehicle, speed at which the vehicle crosses the bridge, and the impact loading of the vehicle on the bridge"
(David Doyle)
and from Greensix.org:
What are Bridge plates?
Bridge plates denote the weight class of bridge that the vehicle may cross. Sometime the signs had a number above another number. This mainly applied to cargo vehicles and was the unladen weight bridge class over the fully laden bridge class
For single vehicles
- yellow circular sign with black numerals
- sign is painted or attached to front or side of vehicle below drivers line of vision
- signs on front of vehicle are 9" in diam, side mounted plates are 6" in diam
For combo vehicles (semi-trailer)
- any tow or trailer that operate as one complete unit or rig
- the front plate must have a RED "C" above the bridge class number
- side mounted plates on a combo show separate bridge class numbers on 6" sign
What is the Bridge Classification of my vehicle?
There is an expedient formula to use.....
W = (A*P*N)/2000 where
W = gross weight of vehicle
A = average tire contact area in square inches (tires on hard surface)
P = tire pressure in PSI
N = number of tires
then round the number up to nearest whole number
You can also hazard a good guess by taking the absolute published maximum load for your vehicle in pounds, and rounding that figure up to the nearest ton. So, if your vehicle (presumably a GS model) weighs 13,500 lbs, and is allowed a 2 1/2-ton load - 5,000 lbs (check your manual) - your all-up weight is 18,500 lbs... rounded up to the nearest ton, 20,000 lbs, or 10 tons - Bridge Class 10.