Suppose you have a bridge. You must design it so it can handle the maximum weight expected. You must not design it with a simple Gaussian distribution in mind.<p>For example, a small bridge in a seldom used road in the countryside might have a few cars per hour. That&#x27;s a load of a couple of tons per car. But cars density isn&#x27;t randomly distributed. If one is slow, then there might be a lineup of 4 cars in a row, and that&#x27;s more likely then a simple calculation based on 1-(1-p)^4 where p is the single car density.<p>Then twice a day there&#x27;s the school bus. And there&#x27;s the occasional cattle truck, and semi, and anhydrous tanker. So you might have a school bus with a couple of heavy vehicles behind it, as a possible maximum design load.<p>The odds of this can&#x27;t be described with a simple Gaussian, so standard deviations make no sense.<p>Of course, there&#x27;s no need to go crazy overboard and design all bridges to handle a convoy of M1 Abrams tanks. That&#x27;s why rural bridge might have a posted weight limit of, say, 10 tons, with only 1 truck allowed at a time. Even then, some people will push the limits, which is why there&#x27;s a safety factor.<p>For example <a href="http:&#x2F;&#x2F;blogs.mcall.com&#x2F;roadwarrior&#x2F;2014&#x2F;08&#x2F;wehrs-mill-bridge-reopens-with-four-ton-weight-limit.html" rel="nofollow">http:&#x2F;&#x2F;blogs.mcall.com&#x2F;roadwarrior&#x2F;2014&#x2F;08&#x2F;wehrs-mill-bridge...</a> describes a wooden bridge which had a 10 ton limit, until some dumbass fuel tanker weighing 38.2 tons went through. Now the rated limit is 4 tons because of the structural damage.<p>So it&#x27;s not &quot;the maximum that was ever recorded in the dataset of all rural bridges&quot; but &quot;the maximum expected for this bridge&quot;, along with efforts to restrict higher loads.