Final answer:
Weight limits on bridges are often based on axle count because more axles help distribute a vehicle's weight across a greater area, reducing the chance of structural damage. This ensures the bridge can safely support the load without risk to its stability.
Step-by-step explanation:
Weight distribution affects bridge stability, which is why weight limits on small bridges sometimes depend on the number of wheels or axles a crossing vehicle has. Each additional axle typically distributes the vehicle's weight over a larger area of the bridge, reducing the pressure on any single point. This can prevent damage to certain bridge components that might not withstand the force of a heavy vehicle's weight all on a single axle.
Furthermore, bridges are designed with a maximum total load in mind, which encompasses the combined weight of the bridge itself and any vehicles or objects on it. An uneven distribution of weight or too much weight concentrated in a single area can lead to structural failures or long-term damage.