The speed limit is commonly set at or below the 85th percentile operating speed (being the speed which no more than 15% of traffic exceeds), and in the US is frequently set below that speed. Thus, if the 85th percentile operating speed as measured by a "Traffic and Engineering Survey" exceeds the design speed, legal protection is given to motorists traveling at such speeds (design speed is "based on conservative assumptions about the driver, the vehicle, and roadway characteristics"). The theory behind the 85th percentile rules is that, as a policy, most citizens should be deemed reasonable and prudent, and limits must be practical to enforce. However, there are some circumstances where motorists do not tend to process all the risks involved, and as a mass, choose a poor 85th percentile speed. This rule, in practice, is a process for "voting the speed limit" by driving, in contrast to delegating the speed limit to an engineering expert.