<i>You&#x27;re basically saying that everyone should defect because everyone else does.</i><p>So, there are three speeds we need to be concerned with here:<p>1. <i>Design speed</i> -- this is the speed anticipated and planned for by the engineers who designed the road. It&#x27;s visible in features like curves, merging areas and so on.<p>2. <i>Operating speed</i> -- this is the actual speed of traffic on the road once built, and typically is measured as the 85th percentile of observed traffic (i.e., the speed such that 85% of traffic travels at or below that speed).<p>3. <i>Speed limit</i> -- this is the posted maximum above which vehicles can be stopped and ticketed by an enforcement officer.<p>In real-world scenarios both the design speed and the operating speed are often higher than the speed limit, because speed limits are often more strongly influenced -- and always downward, when they are influenced in this way -- by factors other than safety (i.e., politics and revenue).<p>Which is a problem. Good road design anticipates roughly what the operating speed will be, and attempts to match that in the design. Setting a limit which differs significantly from the operating and design speeds is only good for politics (&quot;we&#x27;re making you safer by slowing down the traffic&quot;) and revenue (more tickets issued for speeding); it has no relation to actually-safer roads.<p>And in general, yes, speed differential is more commonly a danger than simple raw speed; the intuitive explanation is that every event of one vehicle passing another creates an opportunity for a collision, and moving at a speed significantly different from all other traffic (regardless of whether faster or slower) increases the number of passing events which necessarily increases the chance of a collision.<p>Thus, regardless of posted limits, it is safer to match your speed to that of surrounding traffic. This is not a case of &quot;everyone should defect because everyone else does&quot;. It&#x27;s a case of &quot;the people who posted the speed limit defected, and you shouldn&#x27;t follow their example&quot;.