Speed Zones

Speed limit determination

By law, the Oregon Department of Transportation (ODOT) is responsible for establishing speed zones on all roads in Oregon.  ODOT administers a Speed Zoning Program through its Traffic-Roadway Section.

A widely accepted principle is to set speed limits as near as practicable to the speed below which 85% of the vehicles are traveling on the highway. Experience has shown that approximately 85% of the motorists drive at a speed that is reasonable and prudent. Speed limits thus established encourage voluntary compliance because they appear reasonable to the public. Those 15% of drivers who will not comply with reasonable speed limits are the drivers who are subject to enforcement action.

The effect posted speed limits have on actual traffic speeds

Posted speed has very little effect on actual traffic speeds. There is a common belief among laymen, and even by some officials, that the mere posting of speed limit signs will cause drivers to react accordingly. This is not true and is why posted speed limits must be realistic to receive compliance.

Unrealistically low speed limits will invite violation by responsible drivers. Enforcement of unreasonably low limits sets up the so-called "speed trap," which results in poor public relations. The posting of proper speed limits has the beneficial effects of smoothing traffic flow and aiding effective law enforcement.

The effect of installing lower speed limit signs

It is a common myth that posting slower speed signs forces drivers to slow down and will result in fewer traffic accidents. National research has shown that the prevailing traffic conditions and the type of street, not the posted speed limit, influence drivers. Generally, speed signs are typically installed at quarter-mile intervals on the major arterial streets and are posted at half-mile intervals on collector streets. Twenty-five mph speed signs are installed at the entrances to subdivisions where the speed zone changes from a higher posting (35 or 45 mph) to the residential speed (25-mph). It is not practical to install speed signs at the end of every residential street.

If an unreasonably low speed is posted, many drivers tend to ignore the signs. There are some drivers who, on the other hand, always try to stay within the posted speed. This can cause conflict between faster and slower drivers, resulting in more accidents. Traffic engineering studies help to determine the prevailing speed of most drivers using a certain street. Additionally, the studies take into account accident records and road conditions. An appropriate speed is then set based upon this data.

Installing stop signs to slow drivers down

Under the right conditions, STOP signs can play an important role in traffic safety. However, STOP signs installed in the wrong place usually create more problems than they solve. Many requests are received for STOP signs to interrupt traffic or slow traffic down. However, studies across the nation show that there are a high number of intentional violations when STOP signs are installed as nuisances or speed breakers.

STOP signs are installed at an intersection only after a careful engineering evaluation of the existing conditions indicates that their installation is appropriate. Four-way Stops are only helpful when traffic volumes are high and close to equal on all approaches to an intersection. For more information on stop signs see our Stop Signs brochure.

Slowing traffic down in neighborhoods

Speeding is typical of a large and diverse family of problems that has a complex set of human responses and reactions at its foundation. People tend to drive at the speed that they feel is safe and appropriate. They are also affected by the speeds that others are driving.

In many cases, the speeders are your neighbors (and possibly, you). Discussions among the neighbors can help to reduce the problem. The County has an educational program available that can provide lawn signs, door hangers, radar reader boards and radar "Smart Carts". The purpose of the program is to advise drivers of their speeds, call attention to inappropriate habits and involve the neighborhood in the process. The available material and radar equipment can be arranged for by contacting (503) 846-7950.

Verification of a speeding problem will be forwarded to the Sheriff's Department so that they can schedule selective enforcement in the area as their resources allow. Under those circumstances, it is helpful to be able to advise the Sheriff's Department as to days and times of day when problem is most noticeable.

In some cases, studies may indicate that speeds are inappropriate for the type of neighborhood and that more significant remedial measures are required. In those cases, a neighborhood traffic management project may be required. Those types of projects require consensus among the neighbors and are limited as to financial resources.

Last, and possibly most important, is our responsibility to drive safely and within the speed limit ourselves. Often, the most important part of the equation is YOU. When we drive safely and appropriately, it has a positive affect on the driving habits of others. The more of us that take that challenge seriously, the greater will be the positive impact on safety within our neighborhoods, and within our community, in general.

Installing speed bumps to slow drivers down

The County does have a program for the installation of speed humps and other traffic calming improvements. The program is a two-phase, two-year process. The first year focuses on passive, less-restrictive measures like educational programs, enforcement, pavement legends, and signing. Should these Phase I actions prove ineffective at reducing excessive speeds or traffic volumes, more restrictive Phase II methods and physical devices may be considered, based on certain threshold criteria.

Under certain conditions, appropriately designed speed bumps can slow traffic within the immediate vicinity of such installations. Such installations are only appropriate on low volume, low speed roadways. As with many other types of controls, improperly designed or inappropriately located speed bumps can have the opposite affect and increase the problem.

Speed bumps must impose reasonable reductions in speed. A speed bump installed on a higher speed road with the expectation of drivers slowing to a 20 -25 mph speed creates the potential for sudden deceleration movements and/or loss of driver control.

There are adverse side-affects of even properly designed and located speed bumps. Studies have shown that drivers tend to accelerate after crossing a speed bump in order to make up lost time. Another side affect, which may be annoying to adjacent homeowners/residents, is the noise resulting from acceleration, deceleration, and vehicle noises (undercarriage, rattling parts, etc.) when crossing the bump. A third side affect is the frequent attempts by drivers to vary their approaches to the bump in order to lessen their impact and potentially increase their comfortable crossing speed.

Flashing lights effect on slowing traffic

When flashers are properly located, they serve a useful function. When they are used improperly and installed in locations where they are not warranted, they soon lose much, if not all, of their effectiveness. More seriously, improper usage greatly reduces the effectiveness of other flashers installed in areas where there is a real need. Above all, it is essential to prove that there is a problem that can be solved through the installation of a flasher before actually employing one. Too often, flashers are installed when someone assumes there is, or is going to be, a problem. It is of the utmost importance that flasher installation beheld to a minimum in order to maintain a high degree of respect for the flasher installations that are truly needed.

Communities in the belief that they will reduce vehicle speeds frequently request flashing beacons (commonly called flashers or flashing lights). Unfortunately, this is not necessarily the case. The following discussion of flashing beacons is offered in the interest of broader public understanding of what flashers can do and what factors must be considered.

Effective Usage of Flashers

A flasher is generally installed at an intersection or in conjunction with a warning sign in advance of an area requiring greater than normal care by the average driver. Flashing beacons serve a useful purpose where the flashing yellow is used to alert drivers to unusual conditions that are not readily apparent, such as: obstructions in the roadway, uncommon roadway conditions, narrow bridges, or unusual conditions hidden from the motorists' view. One of the more common locations where a flasher can be used effectively is at a signalized intersection located just beyond a vertical or a horizontal curve, when the intersection is hidden from the view of approaching travelers. For any flasher to be effective, it must command the respect of the traveling public. In other words, immediately after seeing a flasher, the driver must consistently see an unusual condition that is being singled out for attention. Furthermore, the condition that the driver sees must be viewed as serious enough to justify his having been alerted.

Symptom or Problem

Quite often, community requests for flashers are emotional responses to symptoms, rather than attempts to solve underlying problems. To put this into perspective, let's use an appropriate analogy: the case of measles. Obviously, to cure a patient who has measles, the disease itself (measles) must be treated - not the symptom (rash). In traffic control, it is not uncommon for public responses to be directed at treating symptoms.

In cases where concerned parents are requesting flashers on pedestrian warning signs, a traffic investigation all too frequently reveals that:

  • There is no "safe route to school" plan in the community.
  • There is no pedestrian safety program in the schools.
  • Very young children are allowed to travel to school by whatever route they prefer.
  • Parents are willing to abdicate their responsibilities by placing the entire burden for pedestrian safety on a traffic control device.
  • Local law enforcement officials turn a blind eye to pedestrian traffic violations.
  • Where traffic laws are enforced by conscientious law enforcement officials, parent’s claim that the fault lies in inadequate traffic control devices, not in their children's actions.


The results when flashers are installed when these conditions exist:

  • The flasher soon becomes part of the normal driving environment and is ignored.
  • The community continues to avoid treating the real problem.
  • Drivers conditioned to believe that flashers can be safely disregarded frequently disregard other flashers, which are justified.