Artificially low speed zones risky

February 15, 2013 09:04 am
As the Oregon Department of Transportation’s speed zone investigator for Region 5, I often field questions about how speed zones are established.  So after a recent article appeared in the Observer about the posted speed in Troy, I decided this might be a good time to offer some further clarification.

The Basic Rule Law (ORS 811.100) applies to all roadways in Oregon and is the umbrella under which all other speed laws are subordinate. It requires that, regardless of any posted speed signs, a driver must give due regard to existing conditions and not drive faster than what is reasonable and prudent. Aside from Basic Rule, the law provides two ways for speeds to be set. To distinguish between them, we refer to them generally as “statutory speeds” and “designated speeds.”

A statutory speed is one that is specifically described in the law (Oregon Revised Statutes), such as 15 mph in an alley, 20 mph in a business district, 20 mph in a school zone, 25 mph in a residence district, and 65 mph on most freeway sections. (Incidentally, business and residence districts must meet very specific statutory criteria to be legally considered as such.) Certain residence districts can be posted 15 mph rather than 25 mph, but only if no part of the roadway is wider than eighteen feet.

All other speeds are designated speeds, set under the authority of the state traffic engineer in Salem, after an engineering (speed zone) investigation has been conducted. The state traffic engineer’s authority applies not only to speeds on state highways, but also to county roads and city streets. While designated speeds are provided for in the statutes as well, their details (procedures and conditions) are described in Oregon Administrative Rules, which have the force of law.

In the course of a speed zone investigation to establish a designated speed, many factors are taken into account, some of them objective and some subjective in nature. Those factors include roadway and roadside characteristics, horizontal and vertical alignment (curves, hills, etc.), crash history, volume of traffic, types of vehicles that use the road, pedestrian and bicycle usage, and the 85th percentile speed.

The 85th percentile speed, one of the objective factors, is used the world over for setting speeds, is the most important factor of all, and must be the first thing taken into account before any other factors are considered. It is determined by collecting a large enough sample of free-flowing traffic speeds to make the calculation. Normally, a hand-held device such as a laser or radar gun is used, but rubber tube counters or in-road Automatic Traffic Recorders can also be used, provided they are set up to collect speeds.

The 85th percentile speed

As already stated, the 85th percentile speed is used by roadway authorities in most of the world to set speeds. The engineering principle for it is as follows:

On any given roadway under normal free-flowing traffic and not during adverse weather conditions (regardless of whether or not it has speed signs posted), 85 percent of all motorists will drive at or below the speed that is safe, prudent and reasonable for that road. Fifteen percent will drive above that speed. The idea is to set the speed to the 5 mph increment nearest the 85th percentile, with the goal of obtaining maximum compliance by the prudent drivers, and then the imprudent ones can be ticketed. If the speed is set too far below the 85th percentile, you don’t change driver behavior, you merely increase the number of violators and breed disrespect for speed postings. The only way to get compliance in that situation is to have 24-hour-per-day enforcement, in which case you have an undesirable speed trap.

In fact, the state traffic engineer’s authority to designate speeds is limited to 10 mph below the 85th percentile on city streets, county roads and state highways within city limits. On state highways outside of city limits, that authority is limited to 5 mph below the 85th percentile.

What actually slows the prudent driver down more than a sign with a speed on it (except in the case of rigorous enforcement), is what we call “roadside culture.” In other words, do the surroundings make it appear to the driver that he/she should be going slower? Does the road look narrow (even if it’s not)? Is there a lot of development that makes the area look more urban (buildings, driveways, cross streets, bulbouts, sidewalks, etc.)? Does the road have a lot of curves or hills? Is there a high volume of traffic, or maybe parked cars? Are there trees and shrubs that encroach on the roadway and limit visibility?

Conversely, if a road is wide, straight and flat, drivers have a tendency to speed up.

Extensive research has shown that speeds that are set artificially low are actually less safe than speeds that are properly set. This is probably because some otherwise prudent drivers become less so when they get frustrated and make unwise choices that put them in conflict with slower drivers that are only afraid of getting a ticket.

The 55 mph postings on rural roads (non-freeway) don’t really fall under either the “statutory” or “designated” speeds as described above. Those postings are in place because the Basic Rule Laws (ORS 811.100 and ORS 811.105) provide that exceeding 55 mph on those roadway sections is “prima facie evidence” of having violated the Basic Rule. That prima facie evidence applies even if no 55 mph signs have been posted.

As for unpaved roads, we don’t conduct speed zone investigations on gravel or dirt roads to establish designated speeds on them, because the surface conditions change so drastically and so often. Gravel roads should necessarily be governed only by Basic Rule.


About the author

Richard T. Heinemann, 57, of La Grande is a traffic investigator. He works for the Oregon Department of Transportation in La Grande. My Voice columns should be 500 to 700 words. Submissions should include a portrait-type photograph of the author. Authors also should include their full name, age, occupation and relevant organizational memberships. 


Send columns to La Grande Observer, 1406 5th St., La Grande, Ore., 97850, fax them to 541-963-7804 or email them to This e-mail address is being protected from spam bots, you need JavaScript enabled to view it