Traffic Signals

Traffic Signal Standard DrawingsTraffic signal



A roundabout is a circular intersection with the following characteristics:

  • Entering vehicles slow down and YIELD to vehicles in the intersection.
  • Vehicles must travel at slower speeds, increasing safe travel through the intersection.
  • Traffic circulates counterclockwise through the intersection which reduces conflict and the number of decision points for users.

Roundabouts-Another Safe Intersection brochure.
YouTube video produced by the county demonstrating the operation of a roundabout.

How the Flashing Yellow Arrow works

Hundreds of flashing yellow arrows at left turn signals have been installed around the county. It is important that you know how to properly navigate through an intersection that has these signals installed, whether you are driving a vehicle or riding your Flashing Yellow Arrowbicycle.

When the flashing yellow arrow is displayed, left turns are allowed but drivers and bicyclists must wait behind the stop bar/crosswalk while watching for an appropriate gap in traffic. Drivers and bicyclists will then need to yield to oncoming motorists, bicyclists and pedestrians before making the left turn.

Waiting behind the stop bar/crosswalk during a flashing yellow arrow interval will also ensure that your vehicle or bicycle is detected by the signal’s sensor equipment, which will provide a protected green arrow at the end of the cycle or at the start of the next cycle in case a gap in traffic never becomes available. 

Motorists and bicyclists should not wait in the intersection for the traffic gap, as that is considered obstructing cross traffic which is a citable traffic offense (ORS 811.290).

Learn more about flashing yellow arrows:



When traffic signals should be installed 

Traffic signals should be installed when they will alleviate more problems than they will create. This must be determined on the basis of an engineering study.

A warranted traffic signal that is properly located and operated may provide for more orderly movement of traffic, and may reduce the occurrence of certain types of accidents. On the other hand, an unwarranted traffic signal can result in increased delay, congestion, and accidents.

Many people often believe that traffic signals are the answer to all traffic problems at intersections. This is actually not the case. A traffic signal only functions by stopping traffic and any time a motor vehicle is stopped in the road an accident potential is created. Whether a stop in the roadway is caused by a flat tire, a left turn into a driveway, or by a traffic signal - the possibility always exists that a following motorist will not notice the stopped vehicle until it is too late.

The need for traffic signals should be based on competent engineering studies.






How the County decides where to install traffic signals

Traffic engineers throughout the United States that determine appropriate signal locations use standards referred to as Traffic Signal Warrants. A properly placed and warranted traffic signal can improve the flow of traffic and decrease accidents. An unnecessary traffic signal that doesn’t meet warrants can result in a dangerous intersection for pedestrians, bicyclists, and motorists.

Traffic signals are a tremendous investment. Therefore, the County must carefully prioritize where and when traffic signals will be installed.

When determining whether or not a traffic signal is necessary at a specific location, the County’s Traffic Engineering Section performs an evaluation by asking several questions, which include:






  • How much traffic is there on the intersecting streets?
  • Are high levels of traffic consistent throughout the day or just during a few peak hours?
  • Is there a lot of pedestrian traffic?
  • Is the street a wide, high speed, and busy thoroughfare?
  • Are school children crossing the street?
  • Will a signal improve the flow of traffic or cause gridlock with other nearby signals?

The County’s Traffic Engineering Section collects all of the relevant data at a location that is being considered for a traffic signal and compares it to standards that have been established by extensive research and experience.

It is crucial that the traffic signal meets warrants, otherwise you can create a potentially dangerous situation….





  • Traffic signals do not always prevent accidents.
  • Engineering studies have shown that in many instances, accidents increase after a traffic signal is installed.
  • Pedestrians are lulled into a false sense of security because of the new signal.
  • Studies have shown that rear-end collisions often increase.
  • A traffic signal may increase the amount of traffic into and out of your neighborhood because a signal can indicate that the street is a “through-street” even though it may not be.
  • Signals can cause unnecessary delays to drivers during certain times of the day.
  • Delays result in an increase in air pollution.
  • They can cause driver frustration if there is minimal traffic on the major street.

The County’s Traffic Engineering Section wants to always ensure that vehicle, bicycle and pedestrian safety is improved whenever a traffic signal is installed on the county roadway system.






The traffic signal as a traffic control device

Traffic signals are electrically operated traffic control devices, which alternately direct traffic to stop and to proceed. Traffic signals should be used only where lesser forms of control have proven ineffective. It is important that the selection and use of this traffic control device be preceded by a thorough study of traffic and roadway conditions and that the determination of the type of control and method of operation be based on the study data.

The following discussion examines what factors enter into traffic engineers' decisions to install traffic signals. Because there is a common belief that signals are the answer to all traffic problems at intersections, this is offered in the interest of developing broader public understanding about what signals will do and what they won't do.

Advantages of Traffic Signals

Signals offer the maximum degree of control at intersections - they relay messages of both what to do and what not to do. The primary function of any traffic signal is to assign right-of-way to conflicting movements of traffic at an intersection; it does this by permitting conflicting streams of traffic to share the same intersection by means of time separation. By alternately assigning right-of-way to various traffic movements, signals provide for the orderly movement of conflicting flows. They may interrupt extremely heavy flows to permit the crossing of minor movements, which could not otherwise move safely through the intersection. When properly timed, the traffic signal increases the traffic handling capacity of an intersection, and when installed under conditions, which justify its use, it is a valuable device for improving the safety and efficiency of both pedestrian, bicycle and vehicular traffic. In particular, signals may reduce certain types of accidents, most notably the angle (broadside) collision.

Disadvantages of Traffic Signals

While many people realize that traffic signals can reduce the number of angle collisions at an intersection, few realize that signals can also cause an increase in other types of accidents. It has been well documented that other types of accidents, notably rear-end collisions, usually increase when a signal is installed. Normally, traffic engineers are willing to trade off an increase in rear-end collisions for a decrease in the more severe angle accidents, however when there is no angle accident problem at an intersection, there is nothing to trade off and the installation of traffic signals can actually cause a deterioration in the overall safety at the intersection. This situation sometimes prompts the remark, "You mean you won't do anything until somebody gets killed?!" What is not fully understood is that traffic signals are not a "cure-all" and that the primary goal of all traffic engineers is to attain the safest and most efficient traffic flow feasible. In addition to an increase in accident frequency, unjustified traffic signals can also cause excessive delay, disobedience of signals, and diversion of traffic to inadequate alternate routes.

Traffic signals are much more costly than is commonly realized, even though they represent a sound public investment when justified. A modern signal can cost taxpayers between $250,000 and $500,000 to install, depending on the complexity of the intersection and the characteristics of the traffic using it. Of course there is a perpetual cost which is almost never considered - the cost of the electrical power consumed in operating a signalized intersection 24 hours a day. This averages about $1,500 per year.







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