The 85th Percentile Engineering Standard for Setting Speed Limits…

(Excerpt from “Speed Zoning Information A Case of Majority Rule” Institute of Transportation Engineers)

Generally, traffic laws that reflect the behaviour of the majority of motorists are found to be successful, while laws that arbitrarily restrict the majority of motorists encourage violations, lack public support and usually fail to bring about desirable changes in driving behaviour. This is particularly true when it comes to establishing speed limits.

Speed limits are based on several fundamental concepts deeply rooted within the system of government and law:

1. driving behaviour is an extension of social attitude and the majority of drivers respond in a safe and reasonable manner as demonstrated by consistently favourable driving records;
2. the normally careful and competent actions of a reasonable person should be considered appropriate;
3. laws are established for the protection of the public and the regulation of unreasonable behaviour on the part of individuals; and
4. laws cannot be effectively enforced without the consent and voluntary compliance of the public majority.

One important objective in setting a speed limit is to inform drivers of a reasonable and safe maximum speed under normal driving conditions.

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When less than ideal conditions exist, a driver must adjust vehicle speed accordingly as required by provisions of the Manitoba Highway Traffic Act.

It is a long accepted North American practice to recommend and establish speed limits based on the 85th percentile speed, being the speed at or below which 85% of motorists travel, in conjunction with a detailed engineering analysis of other factors such as collision information.  (Known as the 85th percentile rule, or 85th percentile engineering standard)

Circumstances such as curves on the road, visibility restrictions, pedestrian and parking activity, and adjacent land uses (e.g., schools, shopping centres, etc.) are factors that determine the speed at which the vast majority of motorists elect to operate their vehicle.

A speed limit established on such a basis is also referred to as a “credible speed limit” in that the speed limit matches the image that is inspired by the roadway environment and the traffic operating circumstances encountered.

Features of the driving environment that are relevant to a ”credible speed limit” include the roadway width, the number of lanes, lane lining and marking, the presence of adjacent buildings, as well as trees, utility poles and furniture in the boulevard. Long, straight, wide sections of roadways with a smooth surface in an open clear road environment tend to lend themselves to a higher operating speed than is the case where such features are not present.

(Surprisingly absent from that description is the whim of politicians)

Establishing speed limits in this manner has proven to be effective in that it accommodates traffic in a safe and orderly way and enables the Police to focus their enforcement resources toward the 15% of drivers who operate at excessive speeds.  (That is, excessively too high, or excessively too low.  Ironically, enforcement usually only occurs for higher speed, not the lower speed traveller, even though both are proven to be dangerous to safer/more efficient traffic flow)

Such a criterion recognises that the majority of motorists operate their vehicle in a reasonable and prudent manner with due consideration for conditions encountered, including activity into and out of intersecting public streets and approaches as well as the presence of pedestrians and bicyclists on or near the roadway.

By setting speed limits using the 85th percentile speed, the range of speeds is lessened, providing a more uniform flow of traffic. Studies have shown that:

• more collisions occur when the speeds of vehicles are varied with extremely high or low speeds encountered [1];
• setting speed limits lower than that considered reasonable to the majority of drivers encourages disrespect of speed limits in general;
• posted speed limits which are set higher or lower than that dictated by roadway and traffic conditions are ignored by the majority of motorists; and that
• when speed limits are raised or lowered, there is very little impact on motorists’ actual speeds.
  • ¹“U.S. DOT Publication No. FHWA-RD-98-154”, 1998

Safe, “credible speed limits” can be expected to enhance motorists’ compliance to the speed limit, which in turn can result in a reduction in collisions than would otherwise be the case.

If a speed limit is not credible, motorists will be inclined to elect to drive at a speed that they perceive to be realistic. If speed limits are perceived as being incredible too frequently, it will challenge the public’s trust in the speed limit system generally. A speed limit can be incredible because the speed limit is either perceived as being too low or as being too high.

The net effect of incredible speed limits is that motorists will increasingly disregard that and consequently with frequent incredible limits, motorists will begin more and more disregarding road rules and regulations.

If it is noticed that drivers are disregarding speed and other regulations, it is often a symptom of the application of improper traffic engineering.