The “85th percentile rule” is commonly used to set speed limits in jurisdictions across the U.S. Modern interpretations of the rule are that it satisfies key conditions needed for safe roadways: it sets speed limits deemed reasonable to the typical, prudent driver, reduces the problematic variance in travel speeds among vehicles, and allows law enforcement to focus on speeding outliers. Authoritative publications regularly assert that the rule came about because early driving surveys often found that drivers moving at or below the 85th percentile of a speed on a given roadway were within one standard deviation of the mean speed for that roadway and were in the low involvement group for traffic incidents. But does this widely used rule for setting speed limits really have such a scientific pedigree? Given debates in cities around the U.S. about competing uses of street space, we examine where this rule of driver-set speed limits actually came from and whether rule developers’ rationales still hold true today. While most observers trace the rule to safety research and a 1964 report, we find that the 85th percentile rule actually emerged decades earlier amidst the nascent traffic engineering profession’s preoccupation with “traffic service” to increase vehicular throughput; and with respect to safety, the rule was explicitly intended as a starting point in speed limit setting, and not the last word.

“第85个百分位规则”通常用于在美国各辖区设置速度限制。该规则的现代解释是，它满足安全道路所需的关键条件：它为一般谨慎的驾驶员设置了合理的速度限制，从而减少了问题车辆之间行驶速度的差异，并使执法部门可以重点关注超速的异常值。权威出版物经常断言该规则的产生是因为早期的驾驶调查经常发现，在给定道路上以等于或低于速度的第85个百分位行驶的驾驶员在该道路的平均速度的标准偏差内，并且属于低参与度组交通事故。但是，这种用于设置速度限制的广泛使用的规则真的有这样的科学血统吗？鉴于美国各地城市的辩论 关于街道空间的竞争用途，我们研究了驾驶员设定的速度限制规则的真正来源，以及规则开发者的理论是否在今天仍然成立。虽然大多数观察者将规则追溯到安全研究和1964年的报告中，但我们发现，第85个百分位规则实际上是在几十年前的事中出现的，这是新兴的交通工程专业对“交通服务”的关注，以提高车辆的通行能力。在安全方面，该规则明确地旨在作为限速设置的起点，而不是硬道理。我们发现，第85个百分位数规则实际上是在几十年前就出现的，这是新兴的交通工程专业对“交通服务”的关注，以增加车辆的吞吐量。在安全方面，该规则明确地旨在作为限速设置的起点，而不是硬道理。我们发现，第85个百分位数规则实际上是在几十年前就出现的，这是新兴的交通工程专业对“交通服务”的关注，以增加车辆的吞吐量。在安全方面，该规则明确地旨在作为限速设置的起点，而不是硬道理。